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Canadian  Instituta  for  Historical  Microraproductions  /  Institut  Canadian  da  nticroraproductions  historiquas 


Technical  and  Bibliographic  Notm  /  Notes  techniques  et  bibliographiques 


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The  copy  filmtd  bar*  has  b««n  raproducad  thanks 
to  tha  ganarosity  of: 

University  of   Toronto, 
Science  6  Medicine   Library 

Tha  imagas  appaaring  hara  ara  tha  bast  quality 
possibia  eonsidaring  tha  condition  and  lagibility 
of  tha  original  copy  and  in  kaaping  with  tha 
filnting  contract  spacifications. 


L'axamplaira  filmi  fut  raproduit  grica  A  la 
g*n*rosit*  da: 

University  of  Toronto, 
Science  6  Medicine  Library 

Las  imagas  suivantas  ont  itt  raproduitas  avac  la 
plus  grand  soin,  compta  tanu  da  la  condition  at 
do  la  nattat*  da  l'axamplaira  film*,  at  an 
conformit4  avac  las  conditions  du  contrat  da 
filmaga. 


Original  copias  in  printod  papar  covars  ara  filmad 
baginning  with  tha  front  covar  and  anding  on 
tha  last  paga  with  a  printad  or  iliustratad  impraa- 
sion,  or  tha  back  covar  whan  appropriata.  All 
othar  original  copiaa  ara  filmad  baginning  on  tha 
first  paga  whh  a  printad  or  iliustratad  impraa- 
sion.  and  anding  on  tha  last  paga  with  a  printad 
or  illuatratad  impraaaion. 


Tha  last  racordad  frama  on  aach  microficho 
shall  contain  tha  symbol  -^>  (moaning  "CON- 
TINUED "I.  or  tha  symbol  Y  (moaning  "END"). 
whichavor  appliaa. 

Maps,  pittas,  charts,  ate.  may  ba  filmad  at 
diffarant  raduction  ratios.  Thosa  too  larga  to  ba 
antiraly  includad  in  ona  axposura  ara  filmad 
baginning  in  tha  uppar  laft  h4nd  cornar,  laft  to 
right  and  top  to  bottom,  as  many  framas  as 
raquirad.  Tha  following  diagrams  iliustrata  tha 
mathod: 


Laa  axamplairaa  originaux  dont  la  couvartura  an 
papiar  ast  imprimis  sont  fllmis  an  commandant 
par  la  pramiar  plat  at  an  tarminant  soit  par  la 
darniira  paga  qui  comporta  una  amprainte 
d'imprassion  ou  d'illustration,  soit  par  la  sacond 
plat,  salon  la  et*:  Tous  las  autras  examplairas 
originaux  sont  filmis  an  commandant  par  la 
pramiAra  paga  qui  comporta  una  amprainta 
d'imprassion  ou  d'illustration  at  an  tarminant  par 
la  darhi*ra  paga  qui  comporta  una  talla 
amprainta. 

Un  daa  symbolas  suivants  apparaitra  sur  la 
darniira  imaga  da  chaqua  microficha.  salon  la 
cas:  la  symbola  — ^  signifia  "A  SUIVRE".  la 
symbols  ▼  signifia  'FIN  ". 

Las  cartaa.  planchas.  tablasux.  ate.  pauvant  Atra 
film*s  i  das  taux  da  rMuction  diff Grants. 
Lorsqua  la  documant  ast  trop  grand  pour  fttra 
raproduit  an  un  saul  clichi.  il  ast  fiiirf A  d  partir 
da  I'angla  suptriaur  gaucha.  da  gaucha  k  drolta, 
at  da  haut  an  bas.  an  pranant  la  nombra 
d'imagas  nicassaira.  Las  diagrammas  suivants 
illustrant  la  mithoda. 


1  2  3 


1 

2 

3 

4 

5 

6 

MICROCOPY  RESOLUTION  TEST  CHART 

NATIONAL  BUREAU  OF  STANDARDS 

STANDARD  REFERENCE  MATERIAL  1010a 

(ANSI  and  ISO  TEST  CHART  No.  2) 


<<^»  ^ 


(^ 


THE 


DEVELOPMENT   OF  THE 
HUMAN    BODY 


A  MANUAL  OF  HUMAN  EMBRYOLOGY 


J.    PLAYFAIR   MCIWURRICH,    A.M.,   PH.D. 


PROFESSOR   OF   ANATOMY   IN   THE    UNIVERSITY   OF 


»N     *    ' 


With  Tttfo  Hundred  and  Seventy  Illustrations 


S-3 


yri 


V. 


PHILADELPHIA 


P.    BLAKISTON'S   SON    &   CO. 

IOI2    WALNUT   STREET 
1902 


*• 

'H 


Copyright,  1902, 
By  p.  ULAKISTON'S  SON  &  CO. 


Press  of  Wm.  F.  Ff  u  4.  Co. 

1220-S4  8AN80M  STREET 
PHILADELPHIA. 


PREFACE. 


The  assimilation  of  the  enormous  mass  of  facts        ch 
constitute  what  is  usually  known  as  descriptive  anato  »y 
Iways  been  a  difficult  task  for  the  student.     Part  of 
difficulty  has  been  due  to  a  lack  of  information  re- 
xdin-  ihe  causes  which  have  determined  the  structure 
and  relations  of  the  parts  of  the  body,  for  without  some 
knowledge  of  the  why  things  are  so,  the  facts  of  anatomy 
stand  as  so  many  isolated  items,  while  with  such  knowl- 
edge they  become  bound  together  to  a  continuous  whole 
and  their  study  assumes  the  dignity  of  a  science. 

The  great  key  to  the  significance  of  the  structure  and 
relations  of  organs  is  their  development,  recognizing  by 
that  term  the  historical  as  well  as  the  individual  develop- 
ment, and  the  following  pages  constitute  an  attempt  to 
present  a  concise  statement  of  the  development  of  the 
human  body  and  a  foundation  for  the  proper  understand- 
ing of  the  facts  of  anatomy.     Naturally,  the  individual 
development  claims  the  major  share  of  attention,  since  its 
processes  are  the  more  immediate  forces  at  work  in  de- 
termining the  conditions  in  the  adult,  but  where  the  em- 
bryological    record  fails    to    afford   the   required    dcxta, 
whethe-  from  its  actual  imperfection  or  from  the  incom- 
pleteness of  our  knowledge  concerning  it,  recourse  has 
been  had  to  the  facts  of  comparative  anatomy  as  afford- 
ing indications  of  the  historical  development  or  evolu- 
tion of  the  parts  under  consideration. 

It  has  not  seemed  feasible  to  include  in  the  book  a  com- 


111 


IV 


PKEFACK. 


plete  list  of  the  authorities  consulted  in  its  preparation. 
The  short  bibliographies  appended  to  ach  chapter  make 
no  pretensions  to  completeness,  but  are  merely  indica- 
tions of  some  of  the  more  important  works,  especially 
those  of  recent  date,  which  consider  the  cjuestions  dis 
cussed.  For  a  very  full  biblioj^raphy  of  all  works  treating 
of  human  embryology  up  to  1893  reference  may  be  made 
to  Minot's  Bibliography  of  Vertebrate  Embryology,  pub 
lished  in  the  "Memoirs  of  tin  Boston  Society  of  Natural 
History,"  volume  iv,  1893.  It  is  fitting,  however,  to  ac- 
knowledge an  especial  indebtedness,  shared  by  all  writers 
on  human  embryology,  to  the  classic  papers  of  His,  chief 
among  which  is  his  Anatomic  menschlichcr  Fivibryonen, 
and  grateful  acknowledgments  are  also  due  to  the  ad- 
mirable text-books  of  Minot,  O.  Hertwig,  and  Kollmann. 

Anatomicai,  Laboratory, 

University  of  Michigan. 
October   1,  1907. 


CONTENTS. 

I'Ai.K. 

I  7    26 

I\TH"l)rCIU>N, 

P/^RT  I.-GENERAL  DEVELOPMENT. 

CHAI'TI'K   I. 
The   SiK-mu.U./.M.n   and   S,.ernu,t..Kcnesis;  the   Ov.wu   and    Its        ^_^^^ 
Maturation  and  l-Vnilization 

CHAPTKR  11. 
Tho  Se«mcntati..n  *.f  the  ( )vuni  and  the  Formation  ..f  the  (-rm        ^^^^ 
Layers 

CHAPTKR  III. 
The  Development  •,.•  the  Ivxternal  Form  of  the  Human  I.:ml)ry...       82   1 1 1 

CHAPTKR  IV, 

The  Medullary  Groove,  Not'  :h(.rd,  and  Mesodeimic  S'.mites,  112   127 

CHAPTKR  \. 
The  Volk-stalk,    Melly  stalk,  and   Fetal   Membranes. 128   160 

PART  II.— ORGANOGENY. 

CHAPTKR  VI. 

The  Development  of  the  Integumentary  System, 161-17.^ 

CHAPTKR  VII. 
The  Development  of  the  Connective  Tissues  and  Skeleton,.  .    .    174-215 

CHAPTKR  VIII. 
The  Development  of  the  Muscular  System, 216-2.39 

CHAPTKR  IX. 
The  Development  of  the  Circulator-  and  Lymphatic  Systems,       240-295 

CTiaPTKR  X. 
The  Development  of  the  Digestive  Tract  and  Glands 296-333 


VI  COXTKN  iS. 

CHAFTI'K    XI.  pAr.K. 

Tlif  DcvflontiicJil  ijf  the  IVricardiuiii  and  I'k-iini  |K'rit<»ncuni, 

tlie  Dia     ••igm  and  the  Spleen, .VH-.VSI 

CHAPTHR   XII. 
The  DcvcU)pmc-nt  of  the  Organs  of  Respiration, 352-359 

CMAI'TF-R   XIII. 
The  Development  of  the   Irinogenital  System  and  the  Supra- 
renal Hodies, 360-393 

CHAPTHR  XIV. 
The  Development  of  the  Nervous  System, 394-453 

CHAPTHR  XV, 
The  Development  of  the  Orj^ans  of  Sin-cial  Sense, 454-500 

CHAPTHR  XVI. 
Post-natal  Development,  501-518 

Index 519-527 


'i   t' 


LIST  OF  ILLUSTRATIONS. 


4. 
6. 


Ovum  f.f  New-lM.rn  ChiUl  with  l-<.lHcle-cclls  _ 
d1"  rluns  Illustrating'  the  Prophases  of  Mitos.s 

)■:.  li.  W  ihon),.  .  . 
Diagrams  lllustratiriK 

(Adafli'l  }  om  h.  li.  H  '/»'"»). 

Human  SpermatuzcHin, 

Spermatozoon  of  Rat, 


(.»i,  r/(H»), 

-  (Aihpti-l  jrom 


PACK. 
19 

?1 


the  Mfiaphasc  and  Anpliascs  of  Mitosis.—      ^^ 


r>pfriii.ii'""'" -  —  •■  ■       ,  J,    „,„,„,,. „pnr^is  aS  Sf         'n  UlITfici 


•n  DilTerent 
m 


28 
28 


29 


ion  Lenhossek), 


Dia«ra„.  lUustnUing  tne  Recl-^ion  of  tL  •  Cbromosomes  During      ^^ 

Spermato«enesis      ■  ■  - 

Four  s     lies  in  the    1  ransf.         turn 

V'""«^'"-"""  "^'^  Rat.-(r<'n  Lenhnsck,, 


fa  Spermatid  into  the 


32 


^•-^J^s^'iSi;s'ie;S'Uiii^:'rs^^»Ss  ^ 


of  Development 


10. 
11. 

12. 

l.V 

14. 
l.S. 

16. 
17. 
18. 

19. 

20. 

21. 


Thirty  Years  of  Age.— (Nug«/), 


Ovum  fn.m  o'vary  ''^ ^^^^'^f  vT  "s  T.f  C  Fight  DaV    aft e^ 
Ovary  of  a  Woman  Nineteen  \ears  ot  Age,  nig  u  x^  j. 


Vlenst  ruati<  >n .— ( Kallmann) , 


35 


40 


Section   through   the  Corpus   Luteum   of  a   Rabbit,   Seventy      ^^ 


Dia^rii&St.^aSnoftheChn,mosomesduring  ^^ 

Mouse.— (-S()6o//a) V  V  '  li  ■ 'tiinUrhfk\  S5 

Stages  in  the  Segmentati.m  of  ■;i»"/''V;'^f  •-^^^^i^'i'^/vm;.)'  56 

Four  stages  in  the  Segmentation  of  the  Ovum  of  a   Mouse. 


58 
60 


Latif^agSin  theSeguientationof  the  Ovumof  a  Bat^-CVan     ^^ 
Bcnedcn),. 


22.  T.o  Stages  in  the  Gastrulation  oiAmmo.us.-^M organ  and 
2.    TranS  Section' or  Xm/.tor  Embryo' with  FiVe  Meso- 

25'.  sSion  throulh  an  Embry.>  Amphil^n    ^  Jon)  of  2^  ^^^X  1 
showing  the   Formation  of  the  C.astral  Mesoaerm.     v""' 

■wig) ■ 


65 

66 
67 


68 


VIII 


LIST    OF    ILLUSTRATIONS. 


V 


FIG.  PACK. 

26.  Longitudinal  Sections  through  luiibryos  of  the  Gecko,  showing 

Gastrulation.— (M'(7/),    69 

27.  Diagrams  Illustrating  the  Formation  of  the  Gastral  Mesoderm 

in  the  Gecko— {Will), 70 

28.  Sections  of  Ova  of  a  Bat  showing  (.4)  the  Formation  of  the 

Kndoderni  and  {H  and  C)  of  the  .\mniotic  Cavity. — {Van 
Hmcilin), 72 

29.  .4,  Side  \iew  of  Ovum  of  Rabl)it  .Seven  Days  Old  {Kollikfr); 

H,  Kinhryonic  Disk  of  a  Mole  (//ca/'c) ;  C,  Fmhryonic  Disk 

of  a  Dog's  Ovum  of  about  Fifteen  Days  {Bonnet), 73 

?>0.  Posterior  Portion  of  ;i  Longitudinal  Section  through  the  Ivni- 

bryonic  Disk  of  a  Mole. — {After  Ilcapc) 74 

.31.   Diagram  Illustrating  Concrescence. — {Duval), 75 

M.  Transverse  Section  of  tlie  lunbryonic  Area  of  a  Dog's  Ovimi  at 

about  the  Stage  of   Development    shown   in    Fig.  29,  ('. — 

{Honnct), 76 

?>?i.   Diagram  of  a  Longitudinal  Section  through  the  Kmbryonic  Disk 

of  a  Mole. — {Hcapc), 77 

.34.  Transverse  Section  through  the   P'mbryonic  Disk  of  a   Ral)l)it. 

—{Ajtcr  Van  licncdcn) ' 78 

.35.  Section  of  Ivnibryo  and  Adjacent  Portion  of  an  Ovum  of  1  mm. 

—{l\tcrs) 82 

,36.   Diagrams  to  show  the  Probable  Relationships  of  the  Parts  in 

the  I-'mbryos  Represented  in  Figs,  28,  C,  and  ,3.i, 8,3 

,37.  Ovum  .Measuring  6  X  4. .5  mm.     The  Left  Half  of  the  Chorion 

has  Been  Removed  to.showtiie  I>;ml)ryo. — {von  Spec),  ....  8,S 
,38.   I'^mbrvo  l.,S4  nun.  in   Length,  from   the   Dorsal  Surface. — {von 

Spec) 85 

,39.   Diagrams  Illustrating  the  Constriction  of  the  Kmbryo  from  the 

Volk-sac, '. 86 

40.  Hnil)ryo  2.5   mm.   Long. — {Allen   Thomson),    87 

41.  Reconstruction  of  Kml)ryo  2.11  mm.  Long. — {Ajter  Etcrnod),.  .      88 

42.  Hnil)ryo  2.5  nun.  Long. — {Kollmann), 89 

43.  Kmliryo  Lg,  2. 15  mm.  Long. — {His), 90 

''■4.  Floor  of  the  Pharynx  of  Enibryo  B,  7  mm.  Long.-  (///,s),  ....      92 

45.  Kmbryo  Lr,  4.2  mm.  Long. — {Mis) 9.3 

46.  Kmbryo  of  from  Twenty  to  Twenty-five  Days. — {Costc) 95 

47.  Kmbryo  9. 1   mm.   Long. — {His),   96 

48.  Kmbryo  Br.^,   1,3.6  mm.  Long.— (///v), 98 

49.  .4,  Itmbrvo  Sj,   15  mm.  Long  (showing  Kctopia  of  the  Heart); 

Ji,  Finbryo  L,,  17.5  mm.  Long.— (///.0 99 

50.  Kinbrvo  \Vt,  23  mm.  Long.— (///.v) 100 

5L   Head 'of  Kmbryo  of  6.9  mm.  -{His) 102 

52.  Face  of  Fvmbryo  of  8  mm.-  (His), 103 

53.  I'ace  of  ICmljryo  after  the  Completion  of  the  I'pper  Jaw.    -{His),  104 

54.  Kmbryo  1.34  mm.  Lf)ng. — {Etcrnod), 113 

55.  Diagram   of  a    Longitudinal  Section   through   an   Kmbryo  of 

1.54  mm. — {von  Spec), ,' 114 

56.  Diagrams  sho.'ing  the  Manner  of  the  Closure  of  the  Medullary 

Groove,    115 

57.  Transverse    Sections    through    Mole    Kmbryos,    showing    the 

Forniaticm  of  the  Xotochord. — (Heapc), 116 

58.  Transverse  Section  througfi  the  Second  Mesodcrmic  Somite  of 

a  Sheep  Kmbryo  3  mm.  Long. — {Honnct), 118 


LIST    OF    ILLUSTRATIONS.  "^ 

PAfiF. 

"lio    Trmsverse  Section  of  an  Embryo  of  2.5  mm    (See  Fig.  42) 
'    •  showing  on  ether  side  of  the  Medullary  Canal  a  Mesoderm  c 

Somite'  the  Intermediate  Cell-mass,  and  the  \  entral  Meso-    ^  ^^^ 

.0    Tra;^setcU<ir:f:^^V.yo;;f4:25mm.att^^^  ^^^ 

M     niai^^irniSSrthe^HS^^^'theOastralMe.^  ^^^ 

,2    Diai^:fin£;:tS''r'|rm.ion    of- the    Amnion    an^    ^^^ 

M    Di.™tS^^'tSl^;SSonhl,el.^^^  ^2 

64  SsverL  Si.,n  tlirough  the  Helly-stalk  of  an  I-.mbryo  of    ^^_ 

65  Transler^c!i!^?ofti;e  lindiiiicaiCord  ,;f  kmi.ryosof  (.1)     ^^^ 
.6.  TwJ&n;^n£t?atStheForma.ionof  Clu;ri^^^  ^^^ 

67.  Two  wVom  the  Chorion  of' a^  LT4r  the ' Fifth    '^^ 
6«    Tnnsverse  Sections  through  Chonomc  \ilh  m  ^^//"e,  *^"\!' 

68.  Transverse  secu  k^^^^  ^^^  Development.-(A,  u7/«cfc  is 

"^^L^lugMl  magnified  than  H,  from  Szymoncrwuz.  Hfrom    ^^^ 

69.  Mati!?  Placenta'  after   Separation'  from'  'the  ' Uterus.'-'CK'o//-    ^^^ 

70    SecthmSroughlhe  Placental  Chorion  of  an' Kmbryo'of  Se'ven    ^^_ 

7,     Dia^rsUigth^Relations  of  tW  Fetal 'Me^branej^:.:.^    148 
72.  Surface  View  of  Half  of  the  Decidua  Vera  at  the  End  ..f  the 

Third  Week  of  Gestation.— (/Co//m<j«n) . .  .^  ...  ■■■^  ■ 

7?    Diatrrammatic  Sections  of  the  Utenne  Mucosa,  ^  m  the  Non 

^"eTnant  Uterus,  and  H,  at  the  Beginning  of  Pregnancy.-     ^.^ 

(Kundrat  and  Engdmann), •.  •  ■  '  • ,  ' ;  '  '  ^'  '  i V  i^c" 

74    Sect  ..n  of  an  Ovum  of  1  mm.     A  Section  of  the  Embryo  Lies 

in  the  Lower  Part  of  the  Cavity  of  the  ( )vum.-(/'  rom  Strahl,     ^  ^^ 

75.  SectUrnVhrough  a  'Placentaof  Seven  Months'  Development.--    ^^^ 

76    Diagrammalic'  Section  '  thr.nigh  '  the  '  Human  Placenta  at   the 

Middle  of  the  Fifth   ^Umth.-^ilrom  I hrtwig,  a]ter  Leopold),   ^^^ 

77.  .4,  Section  of  Skin  from  the  Dorsum  of  Finger  of  an  Embryo  of 

4.5  cm.;  ii,  from  the  Plantar  Surface  of  the  Foot  of  an    ^^^ 
Embryo  of  10.2  cm., :■.,'■'''  c'  lu"  c^i^^i' 

78.  Diagram   showing  the  Cutaneous   Distribution  of  the  Spmal    ^^^ 

79  DiagmmTh^^inrthe  Overlap  of' the  ■///,'  "'V'.'and'v 'intercostal 

Nerves  of  a  Monkey.— (-SVurring/ow), .  .  .  .  ...  ■  .  ■  .  •  •  ■  .  •  •  •  •■  • 

80  Longitudinal  Section  through  the  Terminal  Joint  of  the  Index- 

finger  of  an  Embryo  of  4.5  cm., .... .^. •  • 

81.  Longitudinal  Section  through  the  Nail  Area  in  an  Embryo  ot    ^^^ 

17  cm. — {Okamura), ,  ,- 

82    The  Development  of  a  Hair.— (Ko/Zmann),      -■■■■.■ /.    • 

83.  Lower  Surface  of  a  Detached  Portion  of  Epidermis  from  the 

Dorsum  of  the  Hand. — (BUischko),   .....  ... '  ' 

84.  Milk  Ridge  (wr)  in  a  Human  Embryo    -{Kallius) i/" 


LIST   OF    ILLUSTRATIONS. 


FIG. 

85. 


86. 

87. 

88. 

89. 

90. 
91. 

92. 

9.V 
94. 
95. 

96. 

97. 

98. 

99. 

100. 
101. 
102. 
103. 

104. 

105. 

106. 

107. 
108. 
109. 
110 

m 

112, 


PAGE. 

Sections  through  the  Epidermal  Thickenings  which  Represent 
the  Mammary  Gland  in  Embryos  (.4)  of  6  cm.  and  (H)  of 
10.2cm., I'l 

Secti(m  through  the  Mammary  Gland  of  an  Embryo  of  25  cm.— 

{I'rom  Nagel,  after  Basch), 1  ''2 

Portion   of  the  Center  of  Ossification  of  the   Parietal   Bone 

of  a  Human  Embryo •    ^'^ 

Longitudinal  Section  of  Phalanx  of  a  Finger  of  an  Embryo  of 

3*  Months. — {Szymoncrwicz), 1"6 

The  Ossification  Center  of  Fig    88  More  Highly  Magnified.— 

{Szymonowicz) , ^'J 

The  Ossification  Centers  of  the  Vemm.—(Tcsiut), 1/9 

.4,  Transverse  Section  of  the  Femur  of  a  Pig  Killed  after  Havmg 
Been  Fed  with  Madder  for  Four  Weeks;  B,  the  Same  of  a 
Pig  Killed  Two  Months  after  the  Cessation  of  the  Madder 
Feeding. — {Ajter  llourens) 180 

Transverse  Section  through  the  Intervertebral  Plate  of  the 
First  Cervical  Vertebra  of  a  Calf  Embryo  of  8.8  mm. — 
(r-'roriep),    181 

Longitudinal  Section  through  the  Occipital  Region  and  Upper 

Cervical  Vertebrae  of  a  Calf  Embryo  of  18.5  mm.— (Froriep),    182 

A,  A  Vertebra  at  Birth ;  B,  Lumbar  Vertebra  showing  Secondary 

Centers  of  Ossification. — {Sappey) 186 

A,  Upper  Surface  of  the  First  Sacral  Veretbra,  and  B,  Ventral 
\'iew  of  the  Sacrum  showing  Primary  Centers  of  Ossifica- 
tion.—(5a/)/'e;') '  ">" 

Formation  of  the  Sternum  in  an   Embryo  of  about   3   cm. — 

{Ruge) '89 

Sternum  of  New-born  Child,  showing  Centers  of  Ossification. — 

{Gcgcnbaur) ,    190 

Reconstruction  of  the  Chondrocranium  of  an  Embryo  of  14  mm. 

[Levi) 1^1 

Frontal   Section   through    the   Occipital   and    Upper   Cervical 

Regitms  of  a  Calf  Embryo  of  8.7  mm.— {Froriep) 193 

Diagram  showing  the  Five  Branchial  Cartilages,  /  to  V, 194 

Occipital  Bone  of  a  Fetus  at  Term, 196 

Sphenoid  Bone  from  Embryo  of  3  J  to  4  Months.— (.Su />/><'>'), ._.    19/ 

Anterior  Porticm  of  the  Base  of  th'j  Skull  of  a  6  to  7  Months' 

Embrvo. — (.Ajler  von  Spec), 198 

The  Temporal  Bone  at  Birth.  The  Styloid  Process  and  Audi- 
tory Ossicles  are  not  Represented. — {Poirier), 200 

Diagram  of  the  Ossifications  of  which  the  Maxilla  is  Composed, 
as  seen  from  the  Outer  Surface.  The  Arrow  Passes  through 
the  Infraorbital  Canal.— (/"rom  vrn  Spec,  after  Sappey) 203 

Diagram   showing  the  Categories  to  which  the  Bones  of  the 

Skull  Belong, 205 

The  Ossification  Centers  of  the  Scapula.— (Testut), 20/ 

Reconstruction  of  an  Embryonic  Carpus 209 

The  Ossification  Centers  of  the  Os  Tnnominatum.— (7V.f/«0,- •   210 

Longitudinal  Section  through  the  Joint  of  the  Great  Toe  in  an 

Embrvo  of  4.5  cm. — {.\icolas), 213 

Cross-sections   of    Heart-muscle   Cells   fmm    Pig    Emliryos    of 

(.4)  10  mm.  and  (B  and  Q  20  mm.— (.Macalliim)    217 

Cross-section  of  a  Muscle  from  the  Thigh  of  a  Pig  Embryo  75 

mm.   Long. — {Macallum), 218 


LIST   OF   ILLUSTRATIONS. 


XI 


PACK. 

in    En.bry.)  <.f   13   mm.   shewing  the  Formation  of  the  Rectus    ^^^ 

1,4    PeriS'R5-Sjmbryosof04)T«.;Monthsynd(/^)^ 
114.  i«="^"J'^^,j^^S^„„ti,s^  showing  the  Development  of  the  Penneal    ^^^ 

1 15    Head  orEm^ri;>s1'/rof  Wo  Months  and  \li)  of  Three  M.mths 
'''■  showing  the   Extension  of  the  Seventh   Nerve  upon   the    ^^^ 

1,6.   Dia^-^TC^S'oftl.  Body  and  Um.^/^^     234 

118    Secti?,n7thr7ugh'&')  the  Thigh" and  (M' the  Calf  showing  the 

'''■  ^"'Ses  SuppHed  bV  the  Nery^    ■'^'jf  ^'^""^^'l"/"^        236 
in  Ctmtinuaticm  with  the  Thoracic  Senes.~--{AiijterIiolk)        ^^^^ 

119.  Sect^m  thJough  the  Upper  Part  of  the  Arm  showmg  the  Zones 

Supplied  by  the  Nerves.— (/^<'/fe),..  ■••:••■>  ^   uuiVvrn ' 

120.  Transverse  Section  through  the  Area  '^^fS^^'^^'^J^  ^^^^^'."^^ 

bryos  showing  the  Transformation  of  Mesoderm  Cells  into  the 
Vascular  Cords.— (raw  der  Stricht) ;  '  '  ^i  •  i  '  '  ' 

121.  Surface  View  of  a  Portion  of  the  Area  Vasculosa  of  a  Ch>cK.-    ^^^ 

122    The  \Slar  Areas  of'  Rabbit '  Embryos.'  '  'in  B  "the'  Veins  are 

'     ■  RepSnted  by  Black  and  the  Network  is  Omitted.-(ra»    ^^^ 

Beneden  and  Julin), •■■■;,:    'r^  '  \^    '' e  e'^Ui 

123.  Section  of  a  Portion  of  the  Liver  of  a  Rabbit  Embryo  of  5  mm.-    ^^^ 

(vati  der  Stricht), •  •  •  • : :■  y    '  \^'  ' ;/.' "  ' 

124.  Stages  in  the  Transformation  of  an  Erythrocyte  into  an  Erythro-    ^^^ 

plastid.— (sawder  S/r»c/i/),.  . • ■  •  • A'''yy~ 

\->5    Portion  of  a  Section  from  the  I.iver  of  an  Embryo  Cat  of  2./ 
Vnm.  showing  a  Megacaryocyte  Surrounded  by  Erythrocytes 

in  a  Blood-vessel.— (//ou'c//), ■ :  ■ ;;  '  >', '  : 

1 26  Diagrams  Illustrating  the  Formation  of  the  Heart  in  the  Cuinea- 

pig.— (.4//fr  Strald  and  C anus), •  • V } ;  ■  ; '    ^cn 

127  Heart  of  Embryo  of   2.15  mm.,  from  a  Reconstruction.-(//f-0.    250 
128'.   Heart  of  Embryo  of  4.2  mm.  seen  from  the  Dorsal  Surface.-    ^^^ 

129.  Heaifof 'Embryo  of' 5'm'm'.,"seen'f'rom  in  Front  and  Slightly    ^^^ 
from  Above. — {His), •  ■  •  • ■  •  •  • •  •  •  .■  •  '  •  •    ---, 

IM).  Inner  Surface  of  the  Heart  of  an  Embryo  "^ JL^ ,f "'7^  R  ^i;; 

1.^1     Heart  of  Embryo  of  10.2  cm.  from  which  Half  of  the  Right 

Auricle  has  Been  Removed, •  ■  •■  •  •  •  .-lij' '  '        " 

132.  Section  through  a  Reconstruction  of  the  Heart  of  a  Rabbit  fc,m- 
bryo  of  10. 1  mm.^(Born) ;  u  '  ', n' i  u-*^ 

13,?  Diagrams  of  Sections  through  the  Heart  of  Embryo  Ribbits 
to  show  the  Mode  of  Division  of  the  \  entncles  and  of  the 
Auriculo-ventricular  Orifice, ; '  • :  •  •, ."  '  '  '  i  '  ' 

134  Diagrams  showing  the  Development  of  the  Aunculo-ventncular 
Valves  —(/"row  Hcrtui^,  after  Ge^cnhaur),.  . •  ■ 

135.  Diagrams  Illustrating  the  Formation  of  the  Semilu'  .r  Valves.    ^^^ 

— {Gegenbaur),  ■  •■.  • j^, 

136.  Reconstruction  of  Embryo  of  2.6  mm.— (//;.s1,  .  .  •  ...  •••■••••  ,", 

137.  Diagram  lUuslraling  the  Arrangement  of  the  Brnnchial  Vessels,  262 

138.  Arterial  System  of  an  Embryo  of  10  mm.— {His) 


XII 


LIST   OF    ILLUSTRATIONS. 


FIG.  PAGR. 

l.V).   Diagram  IlUistratins;  the  Changes  in  the  Arrangement  of  the 

Hrancliial  .\rcli  Vessels 265 

140.  Diagram  showing  the  Relations  of  the  Lateral  Branches  to  the 

Aortic  Arches, 266 

141.  Diagram  Illustrating  the  Development  of  the  Umbilical  Arteries,    267 

142.  The  Development  of  the  \'ertebral  Artery  in  a  Rabbit  Kmbryo 

of  Twelve  Days. — {Hochstctter), 269 

143.  Kmbryo  of  13  mm.  showing  the  Mode  of  Development  of  the 

Internal  Mammary  and  Deep  P^pigastric  Arteries. — (Malt),.   270 

144.  Diagrams  showing  an  Early  and  a  Late  Stage  in  the  Develop- 

ment of  the  Arteries  of  the  Arm, 27.^ 

145.  Diagratns  I  lliisi rating  Stages  in  the  Development  of  the  Arteries 

of  the  Leg, 275 

146.  Reconstruction  of  the  Head  \'eins  of  Guinea-jjig  Kmbrvos.  - 

(Salztr) 278 

147.  Diagrams  showing  the  Development  of  the  Su])erior  Vena  Cava,    279 

148.  Diai;ranis  Illustrating  the  Transformations  of  the   Om])halo- 

mesenteric  and  Umbilical  Xeins,.—  (llochsMicr), 282 

149.  A,  The  X'enous  Trunks  of  an  Embryo  of  5  mm.  seen  from  the 

\entral  Surface;  H,  Diagram  Illustrating  the  Transforma- 
tion to  the  Adult  Condition.— (//jj), 28,3 

150.  Diagrams  Illustrating  the   Development  of  the  Inferior  Vena 

Cava,   285 

151.  The  Development  of  the  Arm  Veins  in  the  Rabbit. — (Hocli- 

sMtcr), 287 

152.  The  Fetal  Circulation.— (/■><)»»  Kallmann) Colored,    289 

153.  Diagrams  showing  the  Arrangement  of  the  Lymphatic  Vessels 

in  Pig  Embryos  of  (A)  20  mm.  and  (H)  40  mm.~(Sahin),.  .   292 

154.  Developing  Lvmphatic  Gland  from  the  Axilla  of  an  E^mbryo  of 

Eleven  \Veeks.--(Cliicritz), 293 

155.  Reconstruction  of  the  Anterior  Porticm  of  an  Embryo  of  2.15 

mm.— (///.f), 297 

156.  Reconstruction  of  the  Hind  End  of  an  Embryo  6.5  mm.  Long. — 

(Kcihrl), 298 

157.  View  of  the  Roof  of  the  Oral  Fossa  of  Embryo  showing  the  Lip- 

groove  and  the  Formation  of  the  Palate. — (Wt*), 299 

158.  Transverse  Sections  through  the  Lower  Jaw  showing  the  Forma- 

tion of  the  Dental  Shelf  in  Embryos  of  (.4)  17  mm.  and  (B) 

40  mm.~{Rose), 301 

159.  Section  through  the  Frst  Molar  Tooth  of  a  Rat,  Twelve  Days 

Old.— (roH  Hrunn) 303 

160.  Floor  of  the  Pharynx  of  P'mhryos  of  (.4)   7  and  (H)   10  mm. 

showing  the  Development  of  the  Tongue. — (His), 306 

161.  The  Floor  of  the  Pharynx  of  an  Embryoof  about  20  mm.— (///i^),    307 

162.  Diagram   of  the   Distribution  of  the  Sensory   Nerves  of  the 

Tongue. — (Zander) 308 

163.  An  Obli(|ue  Section  through  the  Mouth  Cavity  jf  an  Embryo 

of  about  16  to  1  7  mm. — (His), 309 

164.  The  Fl(M»r  of  the  Pharynx  of  an  Embryo  of  2.15  mm. — (His),.  .    311 

165.  Reconstructions  of  the  Branchial  EpitlieUal  Bodies  of  Embryos 

of  (.4)  14  mm.  and  (li)  26  mm. — (Fourneux  and  \'crdun),.  .    313 

166.  Thyreoid,  Thyiims  and  Epithelial  Bodies  uf  a  New  burn  Child. 

— (Croschufj), 315 

167.  Diagram  showing  the  Origin  of  the  V'arious  Branchial  Epi- 

thelial Bodies.— (A'y/(«), 316 


LIST   OF    ILLUSTRATIONS. 


Xlll 


FIG. 

168. 

169. 
170. 

171. 


nibryos  of  (A)  4.2 


PAGE. 


Reconstructions       .ne  Digestive  Tract  ol 

mm.  and  (b,  5  mm.— (//ti) •        ■ 

Reconstruction  of  Embryo  of  20  mm. —(.U all j.. ■  ■  •  • 

Reconstruction  of  the  Intestine  of  an  Embryo  of  19  mm      1  he 

Figures  on  the  Intestine  Indicate  the  Prmiary  ^'»ls.— ( ya"). 
Repre^ntation  of  f  e  CoiHngs  of  the  Intestine  m  the  Adult 


318 
320 

:.2i 


Condition.     The  Numbers  indicate   the  Primary  Coils.—    ^^^ 


323 


324 


326 

.327 


{Mali  I, 

172    Ca'cum  of  Embryo  of  10.2  cm., , t^ '  u  ' '  ' 

173:  Reconstruction  of  a  Portion  of  the  Intestine  of  an  Kmhryo 
of  28  mm.,  showing  the  Longitudinal  Folds  from  which  the 
VilU  are  Formed.— (Wrrry), ;\\-  [,-'^'u 

174  Reconstructions  of  the  Liver  Outgrowths  of  Rabbit  Embryos 

of  M)  5  mm.  and  {B)  of  8  mm.— {Hamtnar)  . ■■  ■ 

175  Transverse  Section  through  the  Liver  of  an  Embryo  of  Four 

Montlis  —(Toldl  and  Zuckerkandl) , ■  •  •  ■ „  ■■  •  • 

176.  Transverse  Sections  of  Portions  of  the  Liver  of  (.4)  a  Fetus 

of  Six  Montlis  and  {B)  a  Child  of  Four  \e^rs.- Toldt  and    ^^^ 

Zuckerkandt),   •  •  •  •  •  • ;  ' ;  ',■  ' 

177    Injected  Bile  Capillaries  of  Pig  Embryos  of  (.4)  8  cm.,  (tf)   lo 

cm.,  and  (Q  of  Adult  ng.-~{Uendrickson)     .......    •  ■   3^^ 

178.  Reco.istruction  of  the  Pancreatic  Outgrowths  of  an  Embryo  of 

^    e    . . . ,,-   ^/"//>/^*'^  ....       ..■.•••> 

179  Reconstruction  of  a^Rabbit  Embryo  of  Eight  Days,  with  tlie 

Pericaruial  Cavity  Laid  Open.— (//lO, .  .      ;         ,      „•    •  ■'  „ 

180  Transverse  Sections  of  a  Rabbit  Embryo  showing  the  Divisum 

of  the  Parietal  Recesses  by  the  Omphalo-mesenteric  \  ems. 

181.  Recwistmclion  from  a' Rabbit  Embryo  of  Nine  Days  showing 

tVie  Septum  Transversum  from  Above.— (Aarn),.  .....  ...  • 

182.  Diag-.ams  of  {A)  a  Sagittal  Section  of  an  Embryo  showing  the 

Liver  Enclosed  within  the  Septum  Iransversum;  (B)a. 
Frontal  Section  of  the  Same;  (O  a  Frontal  vSection  of  a 
Later  Stage  when  the  Liver  nas  Separated  from  the  Dia-    ^^^ 

Diagram'sho'wing  the' Position' of  the  Dia'phragm  in  Embryos 

of  Different  Ages.— (.\/a//) .•  '  , '  '  \,  '  '  '  1 '„a 

Diagram    showing    the    Arrangement    of    the    Mesentery    and 
Visceral  Branches  of  the  Abdominal  Aorta  in  .n  Embryo  of 

Six  Weeks.— (ToW/), - 

Diagrams  Illustrating  the  Development  of  the  Great  .Hnentum 

and  the  Transverse  Mesocolon.— (//er/nfg), .  ..... •      ■^■*^ 

ii..„*_.,.  :.,^  ♦!,..  Manner  in  wViiph  the  Fixation  ot  tne 

.  .   346 


331 
335 


336 
337 


83. 


184. 


185. 


342 


344 


186. 
187. 


188. 


189. 
190. 


Dia^ra  ns  lUustraiing  the  Manner  in  which  the  Fixation 
Descending  Colon  {C,  takes  Place, 

Diagrams  showing  the  Development  of  the  (,reat  <>m<?nju"i 
and  its  Fusion  with  the  Transverse  Mesocolon.— (-4 //rr  Alien 
Thomson) , ,■ '  ' 

Sectir  through  the  Left  Laver  of  the  Mtsogastnum  <>;  a 
L-ck  Embryo  of  Ninety-three  Hours,  showing  the  Origin 
of  the  vSpleen.— (ToMfeo/if), ,     ',.  '    '  i   \i- '  i  '  '  ' 

Portion  of  a  Sectitm  thn.ugh  an  Embryo  of  tlic  I'ourtli  \\  eeiv. 

(Toldt),    :  .;    ■, c  /  i\  in 

Reconstruction  of  the  Lung  Outgrowths  of  Emliryos  of  (/I)  w, 
{B)  8.5,  and  (C)  10.5  mm.— {Hi's) 


348 


350 


352 


353 


XIV 


LIST  OF    ILLUSTRATIONS. 


i 


fir 


FIC.  PAGE. 

191.  Diu^rain  of  the  Final  Branches  of  the  Mainnialian  I  ,  .nchi.— - 

(MilUr) ^S5 

192.  Reconstruction  of  the  Opening  into  the  Larynx  in  an  linihryo 

of  Twenty-eight  Days,  Seen  frf)ni   Hehind  and  .Above,  the 
Dorsal  Wall  of  the  Pharynx  Being  Cut  Away.     (Kalliut),. .   3.S^ 

193.  Reconstruction    of    the    Mesenchyme    Condensations     wliich 

Represent  the  Hyoid  and  Thyreoid  Cartilages  in  an  Km- 
hryo  of  Forty  Days. — (Kallius) 357 

194.  Transverse  Section  through  the  Ahdciiiinal  Region  of  a  Rabbit 

Knibryo  of  12  mm. — {Miluilkovic:), 360 

195.  Transverse  Section  through  Chick  Embryo  of  about  Thirty-six 

Hours,  362 

196.  Transverse  Section  of  the  Wolffian  Ridge  of  a  Chick  F^nibryo 

of  Three  Days. —  {Mihalkovicz), 364 

197.  Urinogenital   Apparatus  of  a   Male   Pig   Embryo  of  6  cm. — 

{Mihalkovicz) '. 365 

198.  Diagrams  of  Early  Stages  in  tfie  Development  of  the  Meta- 

nephric  Tubules. — (Haycraft), 367 

199.  Three  Stages  in  the  Development  of  a  I'riniferous  Tubule  of  a 

Rabbit.— (//avcra//), 368 

200.  Transverse  Section  through  the  Abdomina.  Region  of  an  Em- 

bryo of  25  mm. — .  Kcibcl), 370 

201.  Reproductive  Organs  of  a  Female  Embryo  of  Six  Months. 

(Adapted  from  Miliulkmics), 372 

202.  Section  through  the  Testis  and  the   Hroad   Ligament  of  the 

Testis  of  an  P^mljryo  of  5.5  mm. — (Mihalkovicz), 373 

203.  Diagram  of  an  Epithelial  Invagination  of  the  Ovary  of  a  Rabbit. 

— (von  Winiwarter), 375 

204.  Section  of  the  Ovary  of  a  New-bom  Child. — (From  Gegctibaur, 

after  Waldeycr), 375 

205.  Diagrams   Illustrating  the  Transformations  of  the   Miillerian 

and  Wolffian  Ducts. — (Modified  from  Huxley) Colored,    378 

206.  Reconstruction  of  the  Cloacal  Region  of  an  Embryo  of  14  mm. — 

(Keibel) 382 

207.  Reconstruction  of  the  Cloacal  Structures  of  an  Embryo  of  25 

mm. — (Adapted  from  Keibel), 383 

208.  The  External  Genitalia  of  an  Embryo  of  25  mm  — (Keibel), 385 

209.  Diagrams    Illustrating    the     Descent    of    the    Testis.— (A//cr 

liertu'ig) 389 

210.  Section  through  a  Portion  of  the  Wolffian  Ridge  of  a  Rabbit 

Embryo  f)f  6.5  \r\m.—  (Aichel) 391 

211.  Ependymal  Cells  from  the  Spinal  Cord  of  an  Embryo  of  4.25 

mm.—dlis), .' 395 

212.  Diagram    showing  the  Development  of  the  Mantle  Layer  in  the 

Sp'        Cord.— (.SV//a/>rr), 396 

213.  Three      it  ions  through  the  Medullary  Canal  of  an  Embryo  of 

2.5  mm.    -(r()«  Lenhossek), 397 

214.  Cells  from  the  Gasserian  Ganglion  of  a  Guinea-pig  Embryo. — 

(van   delinrlitcn), 398 

215.  Transverse  Sections  through  the  Spinal  Cords  of  Embryos  of 

(.1)  about  l'"our  and  a  Half  Weeks  and  (/>')  about  Three 
Months.— (///>) 402 

216.  Reconstruction  of  the  Brain  of  an  F^mbryo  of  2.15  mm. — (His),    404 

217.  Median  Longitudinal  Section  of  the  Brain  of  an  Embryo  of  the 

Third    Month.— (His) 406 


LIST   OF   ILLUSTRATIONS. 


XV 


FIC. 

218. 
210. 
220. 

221. 
222. 
223. 
224. 

225. 

226. 
227. 

22S. 

229. 

230. 

231. 
232. 

233. 
234. 
235. 
236 
237, 
238 
239 
240 
241 
242 
243, 


408 
409 


411 


H2 


415 


417 


Transverse  Section  through  the  Medulla  Oblongata  of  an  Em- 
hrvo  of  9.1  mm. — (His), •  ■  •  •  •  •  •  • •  •  •  • •  •  ■  • 

Transverse  Section  through  the  Medulla  Oblongata  of  r.n  hni- 

bryo  of  alwut  Eight  Weeks.- (//i^), . . ...  • ■  ■•■■■-  ; 

\  Dorsal  View  of  the  Brain  of  a  Rnbbit  Eml)ryo  of  16  nmi. . 
H,  Medi:;n  Longitudinal  Section  ot  a  Calf  Embryo  of  3  cm.  - 
t.ilihalk;nicz) -.  •  •. ;  \V     W  '  \'  li'  ' 

Di-igram  Representing  the  Differentiation  of  the  Cerebellar 
Cells.— (5f/)a/)(?r),    ,■;■■/'     j-    x / 

Transverse  Section  of  the  Thalamencephalon  of  a     Embryo  ot 

Five  Weeks.-  (Hi j). /    , '  .     x ;  \-"    l'-'  \" 

Dorsal  View  of  the  Brain,  the  Roof  of  the  Lateral  \entricles 
being  Removed,  of  an  Embryo  of  13.6  mm. — (llis), 

Median  Longitudinal  Section  of  the  Brain  of  an  Embryo  of 

13.6  mm.-iHis) •  •  •  ... .  •  • .  •  • ■  •  •  •   "^'^ 

Median  Longuudinal  Section  of  the  Brain  of  an  Embryo  Calf 

of  5  cm.— (Mihalkovicz), .  .  •  • J^l 

Brain  of  an  Embryo  of  the  Fourth  Month, :  •  •  ■  '  '    •    v •   ^'^'^ 

Cereberal   Hemisphere  of  an   Embryo  of  about   the   Seventh 

Month.— (Cunningham), •  • .. „■  • •    '♦^S 

Median  Ltmgitudinal  Section  of  the  Brain  of  an   Ivmbryo  of 

Three    Months.— (Mihalkor  cz) •  .  ■  .  .    ^26 

Median  Longitudinal  Section  of  the  Brain  of  an  Embryo  of  the 

Fifth  Month.— (Mihalkoi'icz), 4^7 

Transverse  Section  through  ihe  Medulla  Oblongata  of  an  Em- 
bryo of  10  mm.,  showing  the  Nuclei  of  Origin  of  the  Vagus 
(.V)  and  Hypoglossal  (A7/)  N.rves.— (//;>), 432 

Diagram  showing  the  Sensory  Cornpcments  of  the  Cranial  Nerves 

of  a  Fish  (Mcnidia).—(H''rrick), 436 

Transverse  Section  through  a  Embryo  Shark  (Scyllium)  of  15 
mm.,  showing  the  Orir  of  a  Sympathetic  GangUon.— 
(Onodi), 442 

Diagram   showing  the  Arrangement  of  the   Neurones  of  the 

Sympathetic  System.— (Adapted  from  Huber), Colored,    443 

Transverse  Section  through  the  Spinal  Card  of  an  Embryo  of 

7  mm.— (His) 445 

Reconstruction  of  the  Sympathetic  System  of  an  Embryo  of 

10.2  mm.— (His,  Jr.), 447 

Section  of  a  Cell  Ball  from  the  Intercarotid  Canglitm  of  Man.— 

(From  Hohm  and  Davidoff,  after  Sckaper) 449 

Accessory  Sympathetic  Organs  of  Zuckerkandl  from  a  New- 
born Child.— (Zuckerkandl), 451 

Diagram  Illustrating  the  Relations  of  the  Fibers  of  the  Olfactory 

Nerve. — (Van  Gchuchien) , 456 

Diagrams  Representing  the   Development  of  a  Circam vallate 

Papilla.— ((7ra/»frg), 458 

Transverse  Section  Passing  through  the  Otocvst  (ot)  of  Em- 
bryos of  (.4)  2.4  mm.  and  (li)  4  mm.— (//lO 460 

Reconstructions  of  the  Otocysts  of  Embrv{)s  of  (.1)  6.9  mm. 

and  (H)  10.2  mm.— (His,  Jr ) 461 

Reconstruction  of  the  Otocyst  of  an  Eml)rv<)  of   13.5  mm. — 

(His,  Jr.),     402 

Reconstruction  of  the  Otocvst  of  an   Embryo  of  22   mm. — 

(His,  Jr.) : 463 


jjyj  LISr   OF    ILLUSTRATIONS, 

PACK. 

244  The  Right  Internal  Ear  cf  an  Kmhryo  ..f  Six  Months  -^  (AWj/mv),    465 

245  Section  of  the  Scala  Mediu  of  the  CiK:hlea  of  a  Ral.h.t  hmhryo 

of  55  mm.— (Haginsky),    •••••;■.••■•■•  ■  ■  ■    •  •  •  •  -  „.;,■  .• -•,•   ^ 

246.  Transverse  Section  through  a  Senucircular  Canal  of  a  Rabbit 

Kmbryo  of  Twenty-four  Days.— (V  on  A..//ifc<'r)     .    .  .  .    .  .  .   40/ 

247.  Diagrammatic  Transverse  Section  through  a  U.il  of  the  <;;"chlea, 

showing  the  Relations  of  the  Scala-.-(/' r.>.?i  (.vrlach)       .  .   468 

248  Semi-diagrammatic  View  of  the  Auditory  Ossicles  of  an  Ivm- 

brvo  of  Six  Weeks.— (S»V6e»maM«), /  v  •  '.r •  ' 

249  Diagrams  Ii   istrating  the  Mode  of  Extension  of  the  Tympanic 

Cavity  Around  the  Auditory  Ossicles,.        . .  . ...  ■  •  •  •  • 

250.  Horizontal  Section   Passing  through  the   Dorsjil  )Vall  of  the 

External  Auditory   Meatus  in  an   Embry.)  of  4.5   cm.  -    ^^^ 

251    Staces  in  the  Development  of  the  Pinna.— (//»«),.  ^    ■     .•    •  •  ■  • 
252.'  fiSy  Stage  in  the  Development  of  the  Lens  in  a  Rabbit  Em-  ^_^ 

253.  RecISu^tS^'of  the   Brain  of  an  ■Eml>ryo' of  Four  Weeks; 

showing  the  Chorioid  Fis.surc. — (//u) ;,:  •  '  r  V ^70 

254.  Horonta"  Section  through  the  Eye  c^  an  Embryo  Pig  of  7  mm  479 
'>\k    ^^tir.n.;  throuirh  the  Lens  (.1)  of  Human  Embryo  of   Ihirty 

255.  S<^-\7,|^^,;;;[;"„^^^  of  Pig  Embryo  "f  -^^^  """  -  ^g, 

256  PosteHor^Inner)  Surface  of  the  Lens  from'  an  Adult  si.owing    ^^^ 

257  RadlarS"mll!;;^hTSS'..fanEmbryoof  19cm^(S,i/0;   485 
\\'^.  PotSn'^fTl'ransv'erse  Section  of  the  Retina  of  a  New-born    ^^^ 

259  Dia^amloS'^Devel.;pment(;ftheRetinal  Elements.-    ^^^ 

260  Dia^^lil^ic'&^tl^fJ^lSectionofthebpticCupandStalk 

ni«i<;inff  throueh  the  Chonoid   I'lssure, ■  •  •  •  ■  •  •  •  ■  •  • 

261.  Tran^s've'f  ictioL  through  the  Proximal  Part  of  the      ptic 

Stalk  of  Rat  Embryos  of  (1)  9  mm.  and  («)    H   nun.        ^^^ 

262.  ReciSSloi  of  a  P.irticin  of  the" Eye  <.f  an  Embryo  of'  U^  ^^^ 
26^  Tra,^vers^Scd<m  through' the  Ciliary  kegion  of  W  Chick  Em-  ^^^ 
264    TraS:;f£rSh^U^ma;^  Region  ofaPigEm:    ^^^ 

26.  SecS\;:;oS."^l^^^f;S'the  I^s^Eyelids^  ^^^ 

bryo  of  vSix  Monihs.-iSclm-i  t^f^er-Seidl), ..  .^  .■■    ■■-■,■ 

266.  Child  and  Man  Drawn  to  the  Same  ^-^f^^ ^■^^^\'^^,  fv 

"Gnru'th    0}   the    liram,"    Contemporary   Senna    ^^irn  ,     .      ^^^^ 

^M    Skin  of  a  New-born  Child  and  of  an  Adult    Man,   Draun  ^^^ 

\ni)roximatelv  the  Same  Scale.— (Hcn^n,  ■  ■  y  '  ;.•  ;.. ' 

(Hcnke) 


THE  DEVELOPMENT 


OP  THB 


HUMAN    BODY 


INTRODUCTION. 

A  little  more  than  sixty  years  ago  (1839)  one  of  the 
fundamental  principles  of  biology  was  established  by 
vSchleiden  and  Schwann  as  the  cell  theory.  According  to 
this,  all  organisms  are  composed  of  one  or  jre  s^  uclural 
units  termed  cells,  each  of  which,  in  multicel.  .^ar  organ- 
isms, maintains  an  individual  existence  and  yet  contri- 
butes with  its  fellows  to  the  general  existence  of  the  in- 
dividual. Viewed  in  the  light  of  this  theory,  the  human 
body  is  a  community,  an  aggregate  of  many  individual 
units,  each  of  which  leads  to  a  certain  extent  an  inde- 
pendent existence  and  yet  both  contributes  to  and 
shares  in  the  general  welfare  of  the  community. 

To  the  founders  of  the  theory  the  structural  units  were 
vesicles  with  definite  walls,  and  little  attention  was  paid 
to  their  contents.  Hence,  the  use  of  the  term  "cell"  in 
coiuiection  with  them.  Long  before  the  establishment 
of  the  cell  theory,  however,  the  existence  of  organisms 
composed  of  a  gelatinous  substance  showing  no  indica- 
tions of  a  definite  limiting  membrane  had  been  noted, 
and  in  1S35  a  French  naturalist,  Dujardin,  had  described 


f 


I 


,g  THE    DKVKIOJ'MENT    OF    THK    HUMAN    l»Ol>Y. 

the  eelatinous  material  of  which  certain  marine  organ- 
isms   (Rhizopoda)   were   composed,   termmg   it   sarcode 
ami  maintaining  it  to  he  the  material  -^-tratum  wh.c 
conditioned  the  various  vital  phenomena  exh.b.tc^  b 
the  organisms.     Later,  in   1846.  a  botanist,  von  Moll, 
observed  that  living  plant  cells  contained  a  snmlar  sub- 
stance  upon  which  he  believed  the  ex:stence  of  the  eel 
as  a  vital  structure  was  dependent,  and  he  bestowed 
upon  this  substance  the  name  protoplasm,  by  which  it 
is  nov,  universally  known. 

By  tlie.c  discoveries  the  importance  originally  attrib- 
uted to  the  cell-wall- was  greatly  lessened    and  m  1864 
Max  Schultze  reformulated  the  cell  theory,  defining  the 
cell  as  a  mass  of  protoplasm,  the  presence  or  absence  o 
a  limiting  membrane  or  cell- wall  being  immaterial.     At 
the'lme  time  the  spontaneous  origination  of  cells  from 
an  undifferentiated  matrix,   believed   to   occur  by   the 
older  authors,  was  shown  to  have  no  existence,  every 
cell  originating  by  the  division  of  a  P'-^^^^f  ;"f  .^J^^ 
fact  concisely  expressed  in  the  aphorism  of  \irchow 

omnis  cellula  a  celluld.  

Interpreted  in  the  light  of  these  results  the  human 
body  is  an  aggregate  of  myriads  of  cells,*-t.  e.,  of 
masses  of  protoplasm,  each  of  which  owes  its  origin  to 
The  dw'sion  of  a  preexistent  cell  and  all  of  which  may 
Le  traced  back  to  a  single  parent  cell-a  fertilized  ovum^ 
But  all  these  cells  are  not  alike,  but  just  ^sm  a  social 
community  one  group  of  individuals  devotes  itsel  to  t^c 
performance  of  one  of  the  duties  requisite  to  the  vn  ell- 
being  of  the  community  and  a:.other  group  devotes  itself 
to  the  performance  of  another- duty,  so  too.  m  thebody, 

"^  iThas  l.en  estimamrTi;;;^  thT^un^ber  of  cells  -jtering  J«to  tlje 
c,.mp..siti.,n  of  the  body  of  an  adult  human  l.en.«  ,s  about  twenty  MX 
million  five  hundred  thousand  nulhons! 


INTKODL'CTION. 


19 


one  group  of  cells  takes  upon  itself  one  especial  function 
and  another  another.     There  is,  in  other  words,  in  the 
c«  U-community  a  physiological  division  oj  labor.     Indeed, 
the   comparison   of   the   cell-community    to   the   social 
community  may  be  carried  still  further,  for  just  as  grada- 
tions of  individuality  may  be  recognized  in  the  indivr'ual, 
the  municipality,  the  state,  and  the  republic,  so  too  in 
the  cell-community  there  are  cells;  tissues,  each  of  which 
is  an  aggregate  of  similar  cells;  organs,  which  are  aggre- 
gates of  tissues,  one,  however,  predominating  and  deter- 
mining the  char^icter  of  the  organ;  and  systems,  which  are 
aggregates    of    organs    having 
correlated  functions. 

It  is  t!  "  province  of  embry- 
ology to  study  the  mode  of  di- 
vision of  the  fertilized  ovum 
and  the  progressive  differenti- 
ation of  the  resulting  cells  to 
form  the  tissues,  organs,  and 
systems.  But  before  consider- 
ing these  phenomena  as  seen 
in  the  human  body  it  will  be  well  to  get  some  general 
idea  of  the  structure  of  an  animal  ce'l 

This  (Fig.  i),  as  has  been  alrea  .ed,  is  a  mass  of 

protoplasm,  a  substance  which  in  ti.  living  condition  is  a 
viscous  fluid  resembling  in  many  of  its  peculiarities  egg- 
albumen,  and  like  this  being  coagulated  when  heat  ■'  or 
when  exposed  to  the  action  of  various  chemical  reagents. 
As  to  the  structure  of  living  protoplasm  little  is  yet  known, 
since  the  application  of  the  reagents  necessary  for  its  accu- 
rate study  and  analysis  results  in  its  disintegration  or 
coagulation.  But  even  in  the  living  cell  it  can  be 
seen  tl;at  the  protoplasm  is  not  a  simple  homogeneous 
substance.     What  is  termed  a  nucleus  is  usually  clearly 


Fir,.  1.  Ovum  OF  New-born 
Child  with  FouucuE- 
cauus. — {Mertem.) 


w 


THK    DEVF.r.OPMF.NT  OP   THE    HUMAN    H()I)V. 


discernible  us  a  more  or  less  spherical  body  of  a  greater 
refractive  index  than  the  surrounding  protoplasm,  and 
since  this  is  a  permanent  organ  of  the  cell  it  is  con- 
venient to  distinguish  tue  surrounding  protoplasm  as  the 
cytoplasm  from  the  nuclear  protoplasm  or  karyoplasm. 

The  study  of  protoplasm  coagulated  by  reagents  seems 
to  indicate  that  it  is  a  mixture  of  substances  rather  than 
a  simple  chtiuical  compound.  Both  the  cytoplasm  and 
the  karyoplasm  consist  of  a  more  solid  substance,  the 
reticulum,  which  forms  a  network  or  felt -work,  in  the 
interstices  of  which  is  a  more  fluid  material,  the  enchy- 
Icma*  The  karyoplasm,  in  addition,  has  scattered 
along  the  fibers  of  its  reticulum  a  peculiar  material 
termed  chromatin  and  usually  contains  embedded  in  its 
substance  one  or  more  spherical  bodies  termed  nucleoli, 
which  may  be  simply  larger  masses  of  chromatin  or 
bodies  of  special  chemical  composition.  And,  finally, 
in  all  actively  growing  cells  there  is  differentiated  in  the 
cytoplasm  a  peculiar  body  known  as  the  archoplasm 
sphere  in  the  center  of  which  there  is  usually  a  minute 
spherical  body  termed  the  cenirosome. 

It  has  been  already  stated  that  new  cells  arise  by  the 
division  of  preexisting  ones,  and  this  process  is  associated 
with  a  scries  of  complicced  phenomena  which  have 
great  significance  in  connection  with  some  of  the  problems 
of  embryology.  When  such  a  cell  as  has  been  described 
above  is  about  to  divide,  the  fibers  of  the  reticulum  in 
the    neighborhood    of    the    archoplasm    sphere    arrange 


*  It  has  t)et'n  observed  that  certain  coagulable  substances  and  gelatin, 
when  subjected  to  the  reaj^ents  usually  employed  for  "fixing"  proto- 
plasm, present  a  structure  similar  to  that  of  protojilasm,  and  it  has  l)een 
held  that  pmiopliisiii  iu  the  uncoagulatcd  condition  is,  like  these  sub- 
stances, a  more  or  less  homogeneous  material.  On  the  other  hand, 
Biitschli  maintains  that  living  protoplasm  has  a  foam-structure  and  is, 
ill  other  woids,  an  euuilsiun. 


INTRODUCTION. 


21 


themselves  so  as  to  form  fibrils  radiating  in  all  directions 
from  the  sphere  as  a  center,  and  the  archoplasm  with 
its  contained  centrosome  gradually  elongates  and  finally 
divides,  each  portion  retaining  its  share  of  the  radiating 
fibrils,  so  that  two  asters,  as  the  aggregate  of  centrosome, 


I'lG.  2.--DIAGRAMS    ImuSTRATlNG   THE    PROPHASES  OF    MtTOSIS — 
(Adapted  from  E.  li.  Wilson.) 

sphere,  and  fibrils  is  termed,  are  now  to  be  found  in  the 
cytoplasm  (Fig.  2,  A).  .  Gradually  the  two  asters  separate 
from  o  -  another  and  eventually  come  to  rest  at  opposite 
sides     .    he  nucleus  (Fig.  2,  C).  •  In  this  structure  im- 


I 


22  THE    DEVELOPMENT   OF    THE    HUMAN    BODY. 

portant  changes  have  been  taking  place  in  the  mean  time. 
The  chromatin,  originally  scattered  irregularly  along  the 
reticulum,  has  gradually  aggregated  to  form  a  contmu- 
ous  thread  (Fig.  2,  A),  and  later  this  thread  breaks  up 
into  a  definite  number  of  pieces  termed  chromosomes  (Fig. 
2   B)    the  number  of  these  being  practically  constant  for 
each  species  of  animal.    Thus,  in  the  mouse,  the  salaman- 
der and  the  trout  the  number  of  chromosomes  is  twenty-^    ; 
four-  in  the  ox,  the  guinea-pig,  and  man  it  is  s«ete^;' 
while  in  one  of  the  round-worms  (Ascaris)  the  number 
,  ay  be  as  small  as  four,  or  even  two.     It  is  to  be  noted 
that  the  number  is  always  an  even  one. 

As  soon  as  the  asters  have  taken  up  their  position  on 
opposite  sides  of  the  nucleus,  the  nuclear  reticulum  begins 
to  be  converted  into  a  spindle-shaped  bundle  of  fibrils 
which  associate  themselves  with  the  astral  rays  and 
have  lying  scattered  among  them  the  chromosomes 
f Fie  2 '  C)  To  the  figure  so  formed  the  term  amphiaster 
s  applied,'and  soon  after  its  formation  the  chromosomes 
arrange  themselves  in  a  circle  or  plane  at  the  equator 
of  the  spindle  (Fig.  2.  D)  and  the  stages  preparatory  to 
the  actual  division,  the  prophases,  are  completed. 

The  next  stage,  the  metaphase  (Fig.  3,  A),  consists  of 
the  division,  usually  longitudinally,  of  each  chromosome, 
so  that  the  cell  now  contains  twice  as  many  chromosomes 
as  it  did  previously.     As  soon  as  this  division  is  com- 
pleted the      laphases  are  inaugurated  by  the  halves  of 
each  chromosome  separating  from  one  another  and  ap_ 
proaching  one  of  the  asters  (Fig.  3,  B),  and  a  group  of 
chromosomes,  containing  half  of  the  total  number  formed 
in  the  metaphase,  comes  to  lie  in  close  proximity  to  eac 
archoplasm  sphere  (Fig.  3,  C).     The  spindle  and  a    ral 
fibers  gradually  resolve  themselves  again  into  the  reticu- 
lum and  the  chromosomes  of  each  group  become  irregular 


-INTRODUCTION. - 


23 


i 


in  shape  and  gradually  spread  out  upon  the  nuclear 
reticulum  so  that  two  nuclei,  each  similar  to  the  one 
from  which  the  process  started,  are  formed  (Fig.  3,  D). 
Before  all  these  changes  are  accomplished,  however,  a 


>: 


s 


Fir,.  3.— Diagrams  Illustrating  the  Metaphase  and  Anphases 
OF  Mitosis.— (.4(/o/>/c<i /row /:.  H.  Wilson.) 

constriction  makes  its  appearance  at  the  surface  of  the 
cytoplasm  (Fig.  3,  C)  and,  gradually  deepening,  divides 
the  cytoplasm  in  a  plane  passing  through  the  equator  of 
the  amphiastcr  and  gives  rise  to  two  separate  cells  (Fig. 

3,  D). 


24 


THE    DEVELOl'MENT    OF     THE    HUMAN    BODY. 


?l 


u 


1 


il 


This  complicated  process,  which  is  known  as  karyo- 
kinesis  or  mitosi.'.,  is  the  one  usually  observed  in  dividing 
cells,  but  occasionally  a  cell  divides  by  the  nucleus  be- 
coming constricted  and  dividing  into  two  parts  without 
any  development  of  chromosomes,  spindle,  etc.,  the 
division  of  the  cell  following  that  of  the  nucleus.  This 
amitotic  method  of  division  is,  however,  rare,  and  it 
seems  probable  that  it  occurs,  as  a  rule,  only  in  cells 
whose  reproductive  activities  are  becoming  impaired. 
In  actively  reproducing  cells  the  mitotic  method  of  divi- 
sion may  be  regarded  as  the  rule. 

Since  the  process  of  development  consists  of  the  multi- 
plication of  a  single  original  cell  and  the  differentiation 
of  the  cell  aggregate  so  formed,  it  follows  that  the  starting- 
point  of  each  line  of  individual  development  is  to  be 
found  in  a  cell  which  forms  part  of  an  individual  of  the 
preceding  generation.     In  other  words,  each  individual 
represents  one  generation  in  esse  and  the  succeeding  gen- 
eration in  posse.     This  idea  may  perhaps  be  made  clear 
by   the   following   considerations.     As   a   result   of   the 
division  of  a  fertilized  ovum  there  is  produced  an  ag- 
gregate of  cells,  which,  by  the  physiological  division  of 
labor,  specialize  themselves  for  various  functions.     Some 
assume  the  duty  of  perpetuating  the  species  and  are 
known  as  the  sexual  or  germ  cells,  while  the  remaining 
ones  divide  among  themselves  the  various  functions  neces- 
sary for  the  maintenance  of  the  individual,  and  may 
be  termed  the  somatic  cells.     The  germ  cells  represent 
potentially  the  next  generation,  while  the  somatic  cells 
constitute  the  present  one.     The  idea  may  be  represented 
schematically  thus : 


I 


INTRODUCTION.  2? 

First  generation 

Somatic  cells  -j-  germ  cells 

i 

Second  generation 
Somatic  cells  f  germ  cells 

Third  generation 
Somatic  cells  -f  germ  cells,  etc. 

It  is  evident,  then,  while  the  somatic  cells  of  each 
generation  die  at  their  appointed  time  and  are  differen- 
tiated anew  for  each  generation  fron.  the  germ  cells,  the 
latter,  which  may  be  termed  collectively  the  ger.,i-plasm, 
are  handed  on  from  generation  to  generation  without 
interruption,  and  it  may  be  supposed  that  this  has  been 
the  case  ab  initio.     This  is  the  doctrine  of  the  continuiiy 
of  the  germ-plasm,  a  doctrine  of  fundamental  importance 
on  account  of  its  bearings  on  the  phenomena  of  heredity. 
It  is  necessary,  however,  to  fix  upon  some  link  in  the 
continuous  chain  of  the  germ-plasm  as  the  starting-point 
of  the  development  of  each  individual,  and  this  link  is 
the  fertilized  ovum.     By  this  is  meant  a  germ  cell  pro- 
duced by  the  fusion  of  two  units  of  the  germ -plasm. 
In  many  of  the  lower  forms  of  life  {e.  g.,  Hydra  and 
certain  turbellarian  worms)  reproduction  may  be  accom- 
plished by  a  division  of  the  entire  organism  into  two 
parts  or  by  the  separation  of  a  portion  of  tJie  body  from 
the  parent  individual.     Such  a  method  of  reproduction  is 
ter.  .ed  non-sexual.     Furthermore  in  a  number  of  forms 
(eg.,  bees,  Phylloxera,  water-fleas)  the  germ  cells  are 
able  to  undergo  development  without  previously  being 
fertilized,  this  constituting  a    method  of  reproduction 
known  as  parthenogenesis.     But  in  all  these  cases  sexual 
reproduction  also  occurs,  and  in  all  the  more  highly  organ- 
ized animals  it  is  the  only  method  which  normally  occurs; 


I  ! 


26  THE    DEVELOPMENT    OF     THE    HUMAN    BODY. 

in  it  a  germ  cell  develops  only  after  complete  fusion  with 
another  germ  cell.     In  the  simpler  ^orms  of  this  process 
little  difference  exists  between  the  two  combmmg  cells, 
but  since  it  is,  as  a  rule,  of  advantage  that  a  certain 
amount  of  nutrition  should  be  stored  up  in  the  germ  cells 
for  the  support  of  the  developing  embryo  until  it  is  able 
to  secure  food  for  itself,  while  at  the  same  time  it  is  also 
advantageous  that  the  cells  which  unite  sha^'  come  from 
different  individuals  (cross-fertilization),  anci  aence  that 
the  cells  should  retain  their  motility,  a  division  of  labor 
has  resulted.     Certain  germ  cells  store  up  more  or  less 
food  yolk,  their  motility  becoming  thereby    impaired, 
and  form  what  are  termed  the  female  cells  or  ova,  while 
others  discard  all  pretensions  of  storing  up  nutrition  and 
are  especiallv  motile  and  can  seek  and  penetrate  the 
inert  ova;  these  latter  cells  constitute  the  male  cells  or 
spermatozoa.     In  many  animals  both  kinds  of  cells  are 
produced  by  the  same  individual,  but  in  all  the  verte- 
brates (with  rare  exceptions  in  some  of  the  lower  orders) 
each  individual  produces  only  ova  or  spermatozoa,  or,  as 
it  is  generally  stated,  the  sexes  are  distinct. 

It  is  of  itnportance,  then,  that  the  peculiarities  of  the 
two  forms  of  germ  cells,  as  they  occur  in  the  human 
species,  should  be  considered. 


U'    '\ 


II  f: 


LITERATURE. 
*E.    B.    Wilson:   "The   Cell   in    Development   and   Inheritance.' 

edition.     New  York,  1900. 
O.  Hertwkv.  "Die  Zelle  und  die  Gewche."     Jena,  1893. 


Third 


I 

ii 


PART  I. 


GENERAL  DEVELOPMENT. 


CHAPTER  I. 

THE  SPERMATOZOON  AND  SPERMATOGEN- 
ESIS; THE  OVUM  AND  ITS  MATURATION 
AND    FERTILIZATION. 

The  Spermatozoon.— The  human  spermatozoon   (Fig. 
4)   is  a  minute  and  greatly  elongated  cell,   measuring 
about  0.05  mm.  in  length  and  co.^r.Isting  of  an  anterior 
broader  portion  or  head   ik)   and  a  narrow  thread-lik- 
tail  (j).     The  head  measures  about  0.005  mm.  in  length 
and  when  viewed  from  one  surface  (Fig.  4,  A)   has  an 
oval  outline,  though  since  it  is  somewhat  flattened  or 
concave  toward  the  tip,  it  has  a  pvriform  shape  when 
seen  in  profile  (Fig.  4,  B).     The  tail  consists  of  several 
portions,     r        ted  immediately  bel.ind   the  head   is  a 
short  cylim         .  portion  1  1    -suring  0.006  mm.  in  length 
which  is  termed  the  middle- piece  or  neck  (m),  and  behind 
this  is  the  flagellum,  of  about  the  same  diameter  as  the 
middle-piece  but  forming  about  four-fifths   (0.04  mm.) 
of  the  entire  length  of  the  spermatozoon.     The  axis  of 
the  flagellum  is  formed   by  a  delicate  filament   which 
projects  somewhat  beyond  the  flagellum,  forming  what 
IS  termed  the  terminal  filament  or  end-piece  (e). 

rJZ^f'''''  ^"  ^^""T  ''^"^"'  P^"^''  ^'^^  spermatozoa  ul  many 
mammalia  possess  a  luml-cap  (Fig.  5,  he)  covering  the  anterior 

27 


I 


It 


I 


28 


THE    DEVELOPMENT    OF   THE    HUMAN    HODY. 


end  of  the  head  and  a  spiral  membrane  wound  around  the 
flaeellum.  The  presence  of  these  structures  has  not  yet  been 
generally  observed  in  the  human  spermatozoon,  though  several 
obsf-rvers  have  claimed  the  existence  of  a  spiral  membrane, 


Im 


1/ 


Fir;.  4— ^HuMAN  Spermatozoon. 

1,   Front  view,  2,  side  view  of  the 

hea.l;  c,   terminal    filament;   k, 

head;    /.tail;   m,   middle-piece. 

{After  Riizius.) 


Fig.  5.— SpSRmatozoOn  of  Rat. 

h,  Head;  he,  head-cap;  mp,  mid 

die-piece;  »i,  neck.— (ypn^ew) 


and  the  head-cap  undoubtedly  exists  in  the  earlier  stages  of 
the  development  of  the  spermatozoon,  though  it  may  later  be 
lost. 

To  understand  the  significance  of  the  various  parts 
entering  into  the  composition  of  tlie  spermatozoon  a 
study  of  their  development  is  necessary,  and  since  the 
various  processes  of  spermatogenesis  have  been  much 
more  accurately  observed  in  such  mammalia  as  the  rat 


SPERMATOGENESIS. 


29 


I 


and  guinea-pig  than  in  man,  the  description  which  fol- 
lows will  be  based  on  what  has  been  described  as  occur- 
ring in  these  forms.  From  what  is  known  of  the  sperma- 
togenesis in  man  it  seems  certain  that  it  closely  resembles 
that  of  these  mammals  so  far  as  its  essential  features  are 
concerned. 

Spermatogenesis.— The  spermatozoa  are  developed  from 
the  cells  which  line  the  interior  of  the  seminiferous  tubules 
of  the  testis.     The  various  stages  of  development  cannot 


Fig.  6.~DiAGRAM  showing  Stages  of  Si-ermatogenesis  as  seen  in 
Different  Sectors  of  a  Seminiferous  Tublue  of  a  Rat 

s,  Sertoli  cell;  ^c',  spermatocyte  of  the  first  order;  sc\  spermatocyte  of 
the  second  order;  jg,  spermatogone;  sp,  spermatid^  ^2,  spermato- 
zoon —{Modified  from  von  Lenhossek.)  ^ 

i 

all  be  seen  at  any  .  ,.  part  of  a  tubule,  but  the  formation 
of  the  spermatozoa  seems  to  pass  along  each  tubule  in 
a  wave-like  manner  and  the  appearances  presented  at 
(liHerent  points  of  the  wave  may  be  represented  diagram- 
niatically  as  in  Fig.  6. 

In  the  first  section  of  this  figure  four  different  genera- 
tions of  cells  are  represented;  above  are  mature  sperma- 
tozoa lying  in  the  lumen  of  tlie  tubule,  while  next  the 


^ 


30 


THE    DEVIXOI'MENT   OK    THE    HUMAN    BODY. 


basement  membrane  is  a  series  of  cells  from  which  a  new 
generation  of  spermatozoa  is  abont   to  develop.     The 
cells  of  this  series  are  of  two  kinds;  the  larger  one  (s) 
will   develop  into  a  structure  known  as  a   Sertoli  cell, 
while  the  others  are  parent  cells  of  spermatozoa  and  are 
termed  spermatogonia  isg).     In  the  next  section  the  Ser- 
toli cell  is  seen  to  have  become  considerably  enlarged, 
its  cytoplasm  projecting  toward  the  lumen  of  the  tu- 
bule,   and   in   the   third   section   the    enlargement    has 
increased    to   such   an   extent   that  the   spermatogonia 
are   forced  away  from   the   basement  membrane,  with 
which   the    Sertoli  cell   alone    is    in    contact.     In    the 
fourth  section  the  spermatogonia  are  seen  in  process  of 
division;  one  of  the  cells  so  formed  will  persist  as  a 
spermatogone,  while  the  other  forms  what  is  termed  a 
primary  spermatocyte  isc^).     The  results  of  the  division 
are  seen  in  the  last  section,  where  four  spermatogonia 
are  seen  again  in  contact  with  the  basement  membrane 
and  above  them  are  four  primary  spermatocytes.      Re- 
turning now  to  the  first  and  second  sections,  the  layer 
of  primary  spermatocytes  may  still  be  seen,  indications 
of  an  approaching  division  being  furnished  by  the  ar- 
rangement of  the  chromatin  in  those  of  the  second  sec- 
tion, and  in  the  third  section  the  division  is  seen  in  pro- 
gress,  the  two  cells  which  result  from  it  being  termed 
secondary    spermatocytes    isc^).     These    cells    almost  im- 
mediately undergo  division,  as  shown  in  the  fourth  sec- 
tion,  each   giving  rise  to  two  spermatids   (sp),   each  of 
which  becomes  later  on  directly  transformed  into  a  sper- 
matozoon (sz).     From  the  primary  spermatocyte  there 
have  been   formed,  therefore,  as  the  result  of  two  mi- 
toses, four  cells,  each  of  which  represents  a  spermatozoon. 
During  these  divisions  important  departures  from  the 
typical    method    of    mitosis    occur.      These    departures 


Sl'ERMATOfJENESIS. 


have  been  most  thoroughly  studied  in  the  lower  form 
but  it  is  probable  that  they  are  fundamentally  simila: 
in  the  mammalia.     It  has  already  been  pointed  out  (p. 
22)  that  the  number  of  chromosomes  which  appear  dur- 


.    7.— Diagram  Illustrating   tjie   Reduction  of   the  Chromo- 
somes During  Spermatogenesis 
Spermatocyte  of  the  first  order;  sc^,  spermatocyte  of   the  second 
order;  sp,  spermatid. 

jing  the  mitoses  of  the  somatic  cells  is  characteristic  for 
I  the  species.  In  the  division  of  the  primary  spermato- 
Jcytes  the  number  of  chromosomes  which  appear  is  ap- 
fparently  only  half  the  characteristic  number,  but  in  real- 
I  ity  It  is  double  that  number,  since  each  chromosome  is 


\J 


1 


I 


32 


TlIK    DEVKI.OI'MKNT   OF     THK    IIIMAN    IJOOY. 


\ 


really  composed  of  four  elements  more  or  less  closely 
united  to  form  a  tetrad.  During  the  mitosis  each  tetrad 
divides  into  two  dynds,  one  of  which  passes  into  each 
secondary    spermatocyte,    and    these    cells    undergoing 

division   without   the    usual 
reconstruction   of    the    nu- 
cleus,   each    of    the    dyads 
which  they  contain  is  halved, 
so  that  each  spermatid    re- 
ceives a    number   of    single 
chromosomes  equal   to  half 
the    number    characteristic 
for  the  species.     This  reduc- 
tion  of   the    chromosomes    of 
the  germ  cells   may  be  un- 
derstood from  the   annexed 
diagram  (Fig.  7).  which  rep- 
resents  the  spermatogenesis 
of    a    form    whose    somatic 
cells  are  supposed   to    con 
tain  eight  chromosomes. 

The  transformation  of  the 
spermatids  into  spermatozoa 
takes  place  while  they  are 
in  intimate  association  with 
the  Sertoli  cells,  a  number 


Fir..  8.  -FoiR   Stacks    in    thk 
Tkxnsi'ormation  ok  a  Spek- 

MATII>     INTO     THK     SPERMATO- 
ZOON   <»P    A    UaT. 

,1    Archoplasni ;  f,  mass  of  clir.)- 
inatin  wliich  is  later  absorbed 
/,  axial  filament;   /;,  head;  he, 
head-cap;  m/?,  middle-piece. 
(i\>u  [ahIiiissiIc.) 


of  them  fusing  with  the  cyto 
phvsm  of  an  enlarged  Sertoli  cell,  as  shown  in  Fig.  6,  ., 
an<l  probably  receiving  nutrition  from  it.  In  each  sper- 
tnatid  there  is  present,  in  addition  to  the  nucleus  a,> 
archoplasm  sphere  from  which  the  centrosomes  have 
migrated  so  as  to  lie  free  in  the  cytoplasm.  The  detail- 
of  the  transformation  are  still  to  a  certain  extent  under 
discussion,  the  view  here  presented  bei..g  only  one  of  th. 


r^F' 


^ 


i 


SPERMATOGENESIS. 


33 


many  which  have  been  adva.icod  within  recent  years. 
On  the  fusion  of  t^e  spermatid  with  a  Sertoli  cell,  a  deli- 
cate filament  (Fig.  8.  /),  the  beginning  of  the  axial  fila- 
ment of  the  spermatozoon,   appears  in   its  cytoplasm, 
seeming  to  rise   from   the  centrosome  which  lies  at  one 
end  of  it.     The  archoplasm  sphere  (u)  and  centrosome 
migrate  to  opposite  sides  of  the  nucleus,  which  gradually 
assumes  an  excentric  position,  and  the  archoplasm  be- 
comes converted  into  the  head-cap  (he)  while  the  cen- 
trosomes  enlarging  form  the  anterior  portion  or  neck  of 
tlie  middle-piece  (mp),  the  remainder  of  that  structure 
being  formed  from  the  axial  filament   surrounded   by  a 
cytoplasmic  sheath.     As  the  axial  filament  lengthens  the 
cytoplasm  is  drawn  out  with  it  to  form  its  sheath,  the 
terminal  portion  of  the  filament  only  projecting  beyond 
the   sheath   to  form  the  end-piece,  and  the  cytoplasm 
surroundi:;.-  the  nucleus  becomes  reduced  to  an  exceed- 
ingly delicate  layer,  so  that  the  head  of  the  spermato- 
zoon (h)  consists  almost  entirely  of  nuclear  substance 
if  the  head-cap  be  left  out  of  consideration. 

The  homologies  of  the  parts  of  the  spermatozoon  with 
those  of  the  spermatid  may  be  presented  in  tabular  form 
thus: 


Spkrmatid. 
;  .'ucleuj 
Archophism. 
Centrosijme. 

Cytoplasm. 


SncRMATOZOON. 

Head. 

Head -cap. 

Neck  of  middle-piece. 
r  Axial  filament. 
<  Sheath  of  middle-piece. 
^  Sheath  of  tail. 


The  spermatozoon  is,  then,  one  of  four  equivalent  cells, 
produced  by  two  successive  divisions  of  a  primary  sper- 
matocyte and  containing  one-half  the  number  of  chromo- 
somes characteristic  for  the  species. 

The  Ovum.— The  human  ovum  is  a  spherical  cell  meas- 
3 


i' 


f  u 


I 


I 


if 

H 

il 


34 


THE    DKVEI.OPMKM-    OK   TlIK    HUMAN    HODY. 


urinj?  about  0.2  mm.  in  diameter  ami  is  contaiiu-d  within 
a  cavity  situated  near  or  at  the  surface  of  the  ovary  and 
termed  a  Graafian  jollide.  This  follicle  is  surrounded  by  a 
capsule  composed  of  two  layers,  an  outer  one,  the  thaa 
externa,  consisting  of  iihrous  tissue  reseml)lin,n  that  found 


riG     0  -SECTION    THROIGII     I'ORTION    Ol'     AN     ( )VARV    OF     AN     OPOSSUM 

llHddlTys  rir^^irnana)  SMOwiNG  Ova  an»  KoluicuEs  in  Various 

STA(iES    or     DEVEI.OI'MENT. 

b,   Blood-vessel;    dp,  discus   proli-crus;    »m'    stratum    gmnulosum;    o, 
ovum;  s,  stroma;  //;,  tlicca  folliculi. 

in  the  ovarian  stroma,  and  an  inner  one,  the  theca  interna, 
composed  of  numerous  spherical  and  fusiform  cells.  Both 
the  theca?  are  richly  supplied  with  blood-vessels,  the  theca 
interna  especially  "being  the  seat  of  a  very  rich  capillary 
network.  Internal  to  the  theca  interna  there  is  a  trans- 
parent, thin,  and  structureless  hyaline  membrane,  within 


i 


TIIK   OVUM. 


3$ 


wliich  is  the  follicle  proper,  whose  wall  is  formed  l)y  a 
layer  of  cells  termed  the  stratum  (jranulosum  (Fijf.  y,  mg) 
aiul  indosinj,^  a  cavity  tilled  with  an  albuminous  fluid,  the 
liquor  jolliculi.  At  one  point,  usually  on  the  surface 
nearest  the  center  of  the  ovary,  the  stratum  jranulosum 


I'"-   10.  -OviM  i-KOM  Ovary  ok  a   Woman    Tiuktv  Vi;aks  ok    AriR 
",  U,n.n,  radiata;  „,  nucleus;  f,  protoplasmic  zone  of  ovum;  /.v   ,,eri- 
vitfllinc  space;  y   yolk;  c/>,  zona  pellucida.  -  (.Vai;./.)       ' 

is  .i,Teatly  thickened  to  form  a  mass  of  cells,  the  discus 
prolujerus  (dp),  which  projects  into  the  cavitv  of  the  folli- 
He  and  encloses  the  ovum  (o).  U^uallv  but  a  single  ovum 
IS  contamed  in  any  discus,  though  occasionally  two  or 
even  three  niav  occur. 


36 


THE    DEVKLOl'MENT    OF     THE    HUMAN    liODY. 


l     u 


i 


The  cells  of  the  discus  proligcrus  are  for  the  most  part 
more  or  less  spherical  or  ovoid  in  shape  and  are  arranged 
irregularly.     In   the  immediate   vicinity  of   the  ovum, 
however,  they  are  more  columnar  in  form  and  are  ar- 
ranged in  about  two  concentric  rows,  thus  giving  a  some- 
what radiated  appearance  to  this  portion  of  the  discus, 
which  is  termed  the  corona  radiata  (Fig.  lo,  cr).     Imme- 
diately within  the  corona  is  a  transparent  membrane,  the 
zona  pellucida  (Fig.  lo,  zp),  about  as  thick  as  one  of  the  cell 
rows  of  the  corona  (0.02  to  0.024  mm.),  and  presentmg 
a  very  fine  radial  striation  which  has  been  held  to  be  due 
to  minute  pores  traversing  the  membrane  and  contammg 
delicate  prolongations  of  the  cells  of  the  corona  radiata. 
Within  the  zona  pellucida  is  the  ovum  proper,  whose 
cytoplasm  is  more  or  less  clearly  differentiated  mto  an 
outer  more  purelv  protoplasmic  portion  (Fig.   10,  p)  and 
an  inner  deutoplasmic  mass  (y)  which  contains  numer- 
ous fine  granules  of  fatty  and  albuminous  natures.     These 
granules  represent  the  food  yolk  or  deutoplasm,  which  is 
usually  much  more  abundant  in  the  ova  of  other  mammals 
and  forms  a  mass  of  relatively  enormous  size  in  the  ova 
of  birds  and  reptiles.     The  nucleus  of  the  ovum  in)  is 
situated    somewhat   excentrically    in    the    deutoplasmic 
portion  of  the  ovum  and  contains  a  single,  well-defined 

nucleolus. 

A  follicle  with  the  structure  described  above  and  con- 
taining a  fully  grown  ovum  may  measure  anywhere  from 
five  to  twelve  millimeters  in  diameter,  and  is  said  to  be 
"mature,"  having  reached  its  full  development  and  being 
ready  to  burst  and  set  free  the  ovum.  This,  however,  is 
not  yet  mature;  it  is  not  ready  for  fertilization,  but  must 
first  undergo  certain  changes  similar  to  those  through 
which  the  spermatocyte  passes,  the  so-called  ovum  at  this 
stage  being  more  properly  a  primary  oocyte.     But  before 


I 


OVULATION    AND    ITS    RELATION    TO    MENSTRUATION.  37 

describing  the  phenomena  of  maturation  of  the  ovum  it 
will  be  well  to  consider  the  extrusion  of  the  ovum  and  the 
changes  which  the  follicle  subsequently  undergoes. 

Ovulation  and  its  Relation  to  Menstruation.— As  a  rule, 
but  a  single  follicle  near  maturity  is  found  in  either  the 
(.lie  or  the  other  ovar>-  at  any  given  time.  In  the  early 
stages  of  its  development  a  follicle  is  situated  somewhat 
deeply  in  the  stroma  of  the  ovary,  but  during  its  growth 
il  approaches  the  surface  and  eventually  forms  a  marked 
l)romineiice,  only  an  exceedingly  thin  membrane  separat- 
ing the  cavity  of  the  follicle  from  the  abdominal  cavity. 
Tliis  thin  membrane  finally  ruptures,  and  the  liquor  folli- 
culi,  which  is  apparently  under  some  pressure  while  con- 
tained within  the  follicle,  rushes  out  through  the  rupture, 
carrying  with  it  the  ovum  surrounded  by  some  of  the  cells 
of  the  discus  proligerus. 

The  immediate  cause  of  the  bursting  of  the  follicle  is  not 
yet  clearly  understood.  It  has  been  suggested  that  a 
gradual  increase  of  the  liquor  folliculi  under  pressure  must 
in  itself  finally  lead  to  a  rupture,  and  it  has  also  been 
pointed  out  that  just  before  the  maturation  of  the  follicle 
the  theca  interna  undergoes  an  exceedingly  rapid  develop- 
ment and  vascularization  which  may  play  an  important 
part  in  the  phenomenon. 

Xormally  the  ovum  when  expelled  from  its  follicle  is 
received  at  once  into  the  Fallopian  tube,  and  so  makes  its 
way  to  the  uterus,  in  whose  cavity  it  undergoes  its  de- 
velopment. Occasionally,  however,  this  normal  course 
may  be  interfered  with,  the  ovum  cominr  to  rest  in  the 
tube  and  there  undergoing  its  development  and  producing 
a  tubal  pregnancy;  or,  again,  the  ovum  may  not  find  its 
way  into  the  Fallopian  tube,  but  may  fall  from  the  follicle 
into  the  abdominal  cavity,  where,  if  it  has  been  fertilized 
Jt  will  undergo  development,  producing  an  abdominal 


38 


THE    DF.VEI.Ol'MENT    OF     THE    HUMAN    ISODV. 


,  t 
i 

t 


•i 


pregnancy;  and,  finally,  and  still  more  rarely,  the  ovum 
may  not  be  expelled  when  the  (Graafian  follicle  ruptures 
and  yet  may  be  fertilized  and  undergo  its  development 
within  the  follicle,  bringing  about  what  is  termed  an 
ovarian  pregnancy.  All  these  varieties  of  extra-uterine 
pregnancy  are,  of  course,  exceedingly  serious,  since  in 
none  of -them  is  the  fc  us  viable. 

It  was  long  l)elievf ;'  that  ovulation  was  coincident  with 
certain  periodic  chau.-s  of  tlie  uterus  which  constitute 
what  is  termed  menstruation.  This  phenomenon  makes 
its  appearance  at  the  time  of  puberty,  the  exact  age  at 
which  it  appears  being  determined  by  individual  and 
racial  peculiarities  and  by  climate  and  other  factors, 
and  after  it  has  once  appeared  it  normally  recurs  at 
definite  intervals  more  or  less  closely  corresponding  with 
lunar  months  (?.  c,  at  intervals  of  about  twenty-eight 
tlays,  the  extremes  being  from  twenty-four  to  thirty-four 
days)  until  somewhere  in  the  neighborhood  of  the  fortieth 
or  forty-fifth  year,  when  it  ceases. 

The  structural  changes  associated  with  menstruation 
consist  of  a  preliminary  thickening  of  the  walls  of  the 
uterus,  its  mucous  membrane  and  the  subjacent  tissue 
becoming  highly  vascular  and  eventually  congested. 
Later  the  walls  of  the  blood-vessels  degenerate  and  permit 
of  an  escape  of  blood  here  and  there  beneath  the  mucous 
membrane  which,  in  the  areas  overlying  the  effused  blood, 
undergoes  a  fatty  degeneration  and  is  desquamated,  allow- 
ing of  the  formation  of  a  blood-clot  in  the  cavity  of  the 
uterus.  The  hemorrhagic  portion  of  the  process  lasts 
usually  from  three  to  five  days;  at  its  close  a  regeneration 
of  tlie  lost  portions  of  the  mucous  membrane  begins,  and 
when  this  is  completed  a  resting  period  ensues  which  per- 
sists until  near  the  time  of  a  new  menstrual  period. 

The  local  structural  changes  of  the  uterus  are  associated 


OVULATION    AND    ITS    RELATION    TO    MKNSTKL'ATlON. 


39 


with  decided  constitutional  disturbances.  The  pulse, 
blood-pressure,  temperature,  muscular  power,  and  lun^ 
capacity  are  in  general  somewhat  increased  before  men- 
S  struation  and  sink  immediately  before  or  at  the  time 
.  when  the  hemorrhage  in  the  uterus  begins;  innnediately 
iK'fore  the  menstrual  period  there  is  also  a  diminislted 
destruction  of  the  nitrogenous  materials  of  the  body,  as 
shown  by  the  amount  of  nitrogen  excreted  being  less  than 
at  other  times. 

These  general  es  may  well  affect  the  ovar\'  as  well 

as  other  portions    .  iiie  body  and  so  contribute  to  a  coin- 
cidence of  menstruation  and  ovulation.     And,   indeed, 
there  seems  little  question  but  that  the  coincidence  is  of 
fietpient  or  even  usual  occurrence.      The  appearance  of. 
menstruation  indicates,  as  a  rule,  the  beginning  of  fertil- 
ity, and  sterility  ensues  at  the  time  of  the  final  cessation 
of  the  menses.     Furthermore,  menstruation  cca.scs  when 
•  pregnancy  supervenes,  and  the  cessation  persists  not  only 
,  until  parturition,  but  so  long  as  the  child  remains  un- 
^  weaned,  and  as  a  rule  ovulation  is  also  in  abevance  during 
■  I  the  same  period.     Exceptions,  however,  have  been  ob- 
I  served  which  show  that  the  coincidence  of  the  two  phe- 
1  nomena  is  not  invariable,  pregnancy,  for  example,  having 
I  occurred  m  young  girls  who  had  not  yet  menstruated, 
and  m  forty-two  operated  cases  in  which  the  ovaries  and 
uterus   liad   been   remo  ed   after   menstruation,    twelve 
showed  no  .signs  of  ovulation  as  determined  by  the  pres- 
:  once  of  recently  ruptured  follicles  in  the  ovaries  (Leopold 
_  and  Mironoff),  while  in  another  set  of  fiftv-tour  cases 
^  ovulation  appearer'  to  have  coincided  with  menstruation 
;^  in  thirty-nine  instances. 

j  ^'^'"  ^'^«^  evidence  at  present  at  our  disposal  it  mav  be 
,f  stated  that  in  the  human  species  while  ovulation  generally 
I   coincides  with  menstruation,   yet  the  two  phenomena 


40 


THE    OF.Vr.I.Ol'MENT    OF    THE    HUMAN    HODY 


-    d 


may,  and  not  infreqi;ently  do,  occur  independently  of  one 

another. 

The  Corpus  Luteum.— With  the  setting  free  of  the  ovum 
the  use'ihiess  of  the  Graafian  follicle  is  at  an  end,  and  it 
Vri-.s  at  once  to  underj,^o  retrogressive  chan-es  which 
result  primarilv  in  the  formation  of  a  structure  known  as  I 
the  corpus  luteum  (Fis-  1 1  )•  «"  the  rupture  of  the  follicle 
a  considerable  portion  of  the  stratum  granulosum  remains 
in  place,  and  usually  there  is  an  effusion  of  a  greater  or  less 
amount  of   blood  from  the  vessels  of  the  theca  interna 

into  the  follicular  cavity. 
The    split    in    the    wall 
through  which  the  ovum 
escaped  soon  closes  over 
and  the  cavity  becomes 
filled  with  cells  separated 
into    groups    by    trabe- 
cular of  connective  tissue 
containing   blood-vessels 
(Fig.    12).       These  cells 
contain    a    considerable 
amount  of  a  peculiar  yel- 
low  pigment   known   as 
lutein,  the  color  imparted 
to  the  follicle  by  this  substance  having    niggested  the 
name  corpus  luteum  which  is  now  applied  to  it. 

In  later  stages  there  is  a  gradual  increase  in  the  amount 
of  connective  tissue  present  and  a  corresponding  diminu- 
tion of  the  lutein  cells,  the  corpus  luteum  gradually  losing 
its  yellow  color  and  becoming  converted  into  a  whitish, 
fibrous,  scar-like  body,  the  corpus  albicans,  which  may 
eventually  almost  completely  disappear.  These  various 
changes  occur  in  every  ruptured  follicle,  whether  or  nor 
the  ovum  which  was  cc  itained  in  it  be  fertilized.     But 


Fig.  11.— Ovary  op  a  Woman  Xinb- 
TEEN-  Years  ok  Age,  Hight  Days 

AI-TER    MENSTRIWTION. 

J,  Hl<».d-cl(>t;  /,  Graafian  follicle;  //(, 
theca. — (KoUmavn.) 


THE    CORPUS    I.UTEUM. 


4' 


the  rapidity  with  wliich  the  various  stages  of  retrogression 
ensue  difTers  greatly  according  to  whether  pregnancy 
occurs  or  not,  and  it  i^  customary  tc  distinguish  the  cor- 
pora lutea  which  are  associated  with  pregnancy  as  corpora 
luted  vera  from  those  whose  o\'a  fail  to  be  fertilized  and 
which  form  corpora  lutca  spuria.     In  the  latter  the  retro- 


SECTIOX    THKOIJUH    THK    CuKPUS    Li;TKI   M    OK    A    RaBHIT, 

Skvextv   Hoiks  po.'it  roitum. 
riic  civity  of  the   follicle  is  almost  completely  tilled  with   lutein   .-ells 
.iiiiong  which  IS  a  certain  amount  of  connective  tissue.     ,'     Mlood- 
\e>sels;  kc,  ovarial  epithelium.      (Sobolta.) 

^     gression  of  the  follicle  is  completed  usually  in  about  three 
jl     weeks,  while  the  corpora  vera  persist  throughout  the  en- 
tire duration  of  the  pregnancy  and  complete  tiieir  retro- 
gression after  the  birth  of  the  child. 

4 


42 


THE    DEVEI.OI'MENT    OF     THE    HUMAN    BODY. 


Two  very  different  views  are  held  as  to  the  origin  of  the 
lutein  cells.  According  to  one,  which  may  be  termed  von 
Baer's  view,  the  cells  of  the  stratum  granulosum  remain- 
ing in  the  follicle  rapidly  undergo  degeneration  and  com- 
pletely disappear,  and  the  lutein  cells  and  connective- 
tissue  trabecular  are  formed  entirely  from  the  cells  of  the 
theca  interna,  which  increase  rapidly  both  in  size  and 
namber.  The  other  view  was  first  advanced  by  Bischoff 
and  may  be  known  by  his  name.  It  is  to  the  effect  that 
the  granulosa  cells  do  not  disintegrate,  but,  on  the  con- 
trary, increase  rapidly  in  number  and  become  converted 
into  the  lutein  cells,  only  the  connective  tissue  and  the 
blood-vessels  being  derived  from  the  theca  interna. 

Which  of  these  two  views  is  correct  is  at  present  uncer- 
tain.    The  majority  of  those  who  have  within  recent 
years  studied  the  formation  of  the  human  corpus  luteum 
have  expressed  themselves  in  favor  of  von  Baer's  theory. 
Sobotta  has,  however,  made  a  thorough  study  of  the  phe- 
nomena in  a  perfect  series  of  mice  ovaries  and  has  demon- 
strated that  in  that  form  the  lutein  cells  are  derived  from 
the  granulosa  cells.     It  would  seem  strange  if  the  lut  .in 
cells  had  a  different  origin  in  two  different  mammals,  and 
the  observations  on  mice  are  so  thorough  that  one  is 
tempted  to  regard  different  results  as  being  due  to  imper- 
fections in  the  series  of  ovaries  studied,  important  steps  in 
the  development  of  the  corpora  lutea  being  thus  over- 
looked.    Still  the  evidence  available  renders  a  resistance 
to  the  temptation  advisable,  and  the  possibility  of  both 
views  being  correct — the  one  in  some  cases,  the  other  in 
others  -must  be  entertained.     Indeed,  it  has  very  re- 
cently been  suggested  that  the  rapidity  with  which  the 
retrogressive  changes  ensue  in  small  animals  compared 
with  larger  ones  may  be  suflicient  to  account  for  marked 
differences  in  the  mode  of  origin  of  the  lutein  cells  in  dif- 


THE  MATURATION  OF  THE  OVUM. 


43 


fercnt  cases.  If  this  possibility  be  accepted,  then  it  may 
be  said  that  the  weight  of  evidence  is  in  favor  of  the  cor- 
rectness of  von  Baer's  views  in  the  case  of  the  human 
species. 

The  iHaturation  of  the  Ovum.— Returning  now  to  the 
ovum,  it  has  been  shown  that  at  the  time  of  its  extrusion 
from  the  Graafian  follicle  it  is  not  equivalent  to  a  sperma- 
tozoon but  to  a  primary  spermatocyte,  and  it  may  be 
remembered  that  such  a  spermatocyte  becomes  convcrtef' 
into  a  spermatozoon  only  after  it  has  undergone  two  divi 
sions,  during  which  there  is  a  reduction  of  the  number  of 
the  chromosomes  to  one-half  the  number  characteristic 
for  the  species. 

Similar  divisions  and  a  similar  reduction  of  the  chromo- 
somes occur  in  the  case  of  the  ovum,  constituting  what  is 
termed  its  maturation.     The  phenomena  have  not  as  yet 
been  ol)served  in  human  ova,  and,  indeed,  among  mam- 
"  mals  only  with  any  approach  to  completeness  in  the 
mouse  (Sobotta) ;  but  they  have  been  observed  in  so  many 
f  otlier  forms,  both  vertebrate  and  invertebrate,  and  pre- 
i  sent  in  all  cases  so  much  uniformity  in  their  general 
j  features,  that  there  can  be  little  question  as  to  their  occur- 
-  rence  in  the  human  ovum. 

In  typical  cases  the  ovum  (the  primary  oocyte)  under- 
goes a  division  in  the  prophases  of  which  the  chro- 
matin aggregates  to  form  half  as  many  tetrads  as 
tlierc  are  chromosomes  in  the  somatic  cells  (Fig.  13,  oc^) 
and  at  the  metaphase  a  dyad  from  each  tetrad  passes 
into  each  of  the  two  cells  that  are  formed.  These  two 
cells  (secondary  oocytes)  are  not,  however,  of  the  same 
size;  one  of  them  is  almost  as  large  as  the  original  pri- 
mary oocyte  and  continues  to  be  called  an  ovum  {oc^), 
while  the  other  is  very  small  and  is  termed  a  polar  globule 
ip).     A  second  division  of  the  ovum  quickly  succeeds 


1 


1. 


1  ^ 


i 


i 


44  THE    DEVELOPMENT   OF     THE    HUMAN    BODV. 

the  first  (Fig.  13,0c'),  and  each  dyad  gives  a  single  chro- 
mosome to  each  of  the  two  cells  which  result,  so  that  each 
of  these  cells  possesses  half  the  number  of  chromosomes 
characteristic  for  the  species.     The  second  division,  like 


oc- 


Vic.      13       -DiX.-.RAM    lULUSTRATINV.    TH.J     REIMICTION    OK    THK    CHROMO- 
SOMES   UURINf.    THE    MaTI'KATION    Ol'    THE    OVIM. 

o   OMiui;  .,c-',  oocyte  of  the  first  generation;  oc^  oticyte  of  the  second 
generation;  />,  polar  globule. 

the  first,  is  unequal,  one  of  the  cells  being  relatively  very 
large  and  constitutiig  the  mature  ovum,  while  the  other 
is  small  and  is  the  second  polar  globule.     Frequently  the 


THE    MATURATION    OF    TFIE   OVUM. 


45 


first  polar  globule  divides  during  the  formation  of  the 
second  one,  a  reduction  of  its  dyads  to  single  chromosomes 
taking  place,  so  that  as  the  final  result  of  the  maturation 
four  cells  are  formed  (Fig.  13),  the  mature  ovum  (o), 
and  three  polar  globules  (p),  each  of  which  contains  half 
the  number  of  chromosomes  characteristic  for  the  species. 
The  similarity  of  the  maturation  phenomena  to  those 
of  spermatogenesis  may  be  perceived  from  the  following 
diagram : 


Oocyte  I 


o 


o 


spermato- 
cyte I 


Oocvtell 


o 


o 


Ovum  O        O  U 


O 


C  J  r^         Spermalo- 

V^  V^V  ..jteii 

\  I' 

V-J     W  \J     O  Spermatids 


Polar  globules 


In  both  processes  the  number  of  cells  produced  is  the  same 
and  m  both  there  is  the  same  reduction  of  the  c'lromo- 
somes.     But  while  each  of  the  four  spermatids  is  func- 
tional, the  three  polar  globules  are  non-functional,  and 
are  to  be  regarded  as  abortive  ova  formed  during  the  pro- 
cess of  reduction  of  the  chromosomes  only  to  undergo 
(..'generation.     In  other  words,  three  out  of  every  four 
potential  ova  sacrifice  themselves  in  order  that  the  fourth 
may  have  the  bulk,  that  is  to  say,  the  amount  of  nutritive 
material  and  cytoplasm  necessary  for  successful  develop- 
ment. ^ 

In  the  mouse,  which  for  the  present  must  be  taken  as 
type  of  the  mammalia,  the  majority  of  ova  show  an  im- 


46 


TIIK    nF.VF.I.OPMKNT    OF     TlIK    HUMAN    ROOV. 


li^   ;■ 


i 


11 


portant  departure  from  the  processes  just  described.  The 
number  of  chromosomes  occurrinj;  in  the  somatic  cells  of 
the  mouse  is  apparently  twenty  four.  The  first  matur- 
ation spindle  (Fij;.  M)  possesses  twelve  chromosomes, 
wiiich  from  analo^'y  with  the  lower  forms  may  be  assiuued 


lM<;    14    OviiM  c)i-  A  MorsE  Showino   the  Mati^kation  Spindle. 

The  ovum  is  cnd-sedhv  tlio  y.on:x  pellucida  (j/>)    to  wliich  the  cells  of 

the  corona  nuliuta  arc  still  attached.      {Sohoitu). 


to  be  tetrads,  and  during  the  mctaphase  each  chromo- 
some divides  transversely,  the  polar  globule  receiving 
twelve  chromosomes,  presumably  dyads,  while  twelve  re- 
main within  the  ovum.  So  far  the  process  is  essentially 
typical,  but  in  90  per  cent,  of  the  ova  examined  this  was 
the  only  maturation  division  which  took  place,  only  one 


THE    FERTIMZATION    OF    THE    OVtM. 


47 


■"% 


polar  globule  beine;  formed.  In  the  remaining,'  lo  per 
cent,  the  second  division  occurred,  the  twelve  chronio- 
sonies  again  dividing  transversely,  so  that  the  second 
polar  gloliule  and  the  ovum  each  recei\ed  twelve  c!iromo- 
sonies  and  the  reduction  was  typical. 

The  occurrence  of  hut  one  maturation  division  in  an 
iiiunense  majority  of  ova  is  dillicidt  to  explain  and  de- 
mands further  study.  Possibly  in  these  ova  the  supposed 
tetrads  are  in  reality  dyads  and  the  reduction  differs  only 
(juantitatively  from  the  typical  process. 

The  Fertilization  of  the  Ovum. — It  is  perfectly  clear  that 
the  reduction  of  the  chromosomes  in  the  germ  cells  cannot 
very  long  be  repeated  in  successive  generations  unless  a 
restoration  of  the  original  number  takes  place  occasion- 
ally, and,  as  a  matter  of  fact,  such  a  restoration  occurs  at 
the  very  beginning  of  the  development  of  each  individual, 
being  brought  about  by  the  union  of  a  spermatozoon  with 
an  ovum.  This  union  constitutes  what  is  known  as  the 
jeriiliZiition  of  the  ovum. 

The  fertilization  of  the  human  ovum  lias  not  yet  been 
observed,    but    the    phenomenon    has    been    repeatedly 
studied  in  lower  forms,  and   a   thorough   study   of   the 
process  has  been  made  on  the  mouse  by  Sobotta,  whose 
observations  are  taken  as  a  basis  for  the  following  account. 
The  maturation  of  the  ovum  is  quite  independent  of 
fertilization,  but  in  many  forms  the  penetration  of  the 
spermatozoon  into  the  ovum  takes  place  before  the  ma- 
turation phenomena  are  completed .     This  is  the  case  with 
the  mouse.     A  spermatozoon  makes  its  way  through  the 
zona  pellucida  and  becomes  embedded  in  the  cytoplasm 
of  the  ovum  and  its  tail  is  quickly  absorbed  by  the  cyto- 
plasm while  its  nucleus  and  probably  the  middle-piece 
persist  as  distinct  structures.     As  soon  as  the  maturation 
divisions  are  completed  the  nucleus  of  the  ovum,  now 


m,km& 


'  1 


tf^  . 


Kir,.  15.-   Six  Stages  in  the  Process  of  Fertiuz.\tion  of  the 

OviM  oi"  A  MorsE. 
■Vfter  the  first  sta-e  fii,'iin.fl  it  is  impossible  to  determine  which  of  the 
two  nuclei  ripresenls  the  male  or  Icmaie  pronucleus,      ck,  Female 
pronucleus;    rk^  and  rk,,  polar  glolniles;    spk,  male  pronucleus.— 
{Sobotta.) 


48 


If! 

v4 


TMI.    FKRTrLIZATION    OF    TIIK    0\  IM. 


49 


M 


ttrimd  till-  ji'tuiilc  pionuclru  l<\^.  1 5,  ck'),  luijjralts  toward 
tlie  conttr  of  tlu-  ovum,  and  is  now  destitute-  ot  an  ardio- 
plasin  sphere  and  i  ntrosonie,  these  structures  havinj^ 
disappeared  after  the  completion  of  the  maturation  divi- 
sions. The  spermato/.oou  nuckus,  vvliich,  after  it  has 
penetrate('  tlie  ovum,  is  termed  the  nid/c  pyouiichus  (spk), 
may  lie  at  lirst  at  ahnost  any  point  in  the  peripheral  part 
of  the  cytoplasm,  and  it  now  bej,dns  to  approach  the 
female  pronucleus,  preceded  by  the  middle  piece,  which 
becomes  an  archoplasm  sphere  with  its  contained  centro- 
somc  and  is  surrounded  by  astral  ra  "*he  two  pro- 

nuclei finally  come  into  contact  near  die  center  of  the 
ovum,  forming  what  is  termed  the  seymeutation  nucleus 
(Fi,i(.   15),  and  the  archoplasm  sphere  and  centrosonie 
•  which    liave    been    introduced    with    the    spermatozoiin 
undergo  division   and    the   two   archoplasm   spheres   so 
formed  mijrratc  to  opposite  poles  of  the  sedimentation 
nucleus,  an  amphiaster  forms  and  the  compound  nucleus 
passes  through  the  various  prophases  of  mitosis.     Since, 
in  the  mouse,  the  male  and  female  protmclei  have  each' 
contributed  twelve  chromosomes,  the  ecpuitorial  plate  of 
the  mitosis  is  composed  of  twenty-four  chromosomes,  the 
number  characteristic  for  the  species  being  thus  restored. 
It  seems  to  be  a  rule  that  but  one  spermatozoiin  pene- 
■  trates  the  ovum.     Many,  of  course,  come  into  contact 
with  it  and  endeavor  to  penetrate  it,  but  so  soon  as  one 
has  been  successful  in  its  endeavor  no  further  penetration 
«>t  others  occurs.     The  reasons  for  this  are  in  most  cases 
obscure;  experiments  on  the  ova  of  invertebrates  have 
shown  that  the  subjection  of  the  ova  to  abnormal  condi- 
aons  winch  impair  their  vitality  favors  the  penetration  of 
more  than  a  single  spermatozoon  {polyspermy),  and    in- 
deed. It  appears  that  in  some  forms,  such  as  the  common 
n^^i{Dtemyctylus),  polyspermy  is  the  rule,  onlv  one  of 


1 


i 


i   i 


! 


V 


50  THE    DEVELOPMENT    OF    THE    HUMAN    nOOY. 

the  spermatozoa,  however,  whieh  have  penetrated  uniting 
wilh  the  female  pronueleus,  the  rest  being  absorbed  by 
the  cvtor)hism  of  the  ovum. 

'  Fertilization  marks  the  beginning  of  ^^^^^P-"^^ 
it  is  therefore  important  that  something  ^^^^^^  be)"«;^ 
as  to  where  and  when  it  oceurs.  It  seems  probable  that 
in  he  human  speeies  the  spermatozoa  usually  come  mto 
cotitact  with  the  ovum  and  fertilization  ensues  m  the 
upper  part  of  the  Fallopian  tubes,  and  the  oeeurrenee 
^extra-uterine  pregnaney  (see  p.  38)  seems  to  indieate 
that  occasionally  the  ovum  may  be  fertilized  even  before 
it  has  been  received  into  the  tube.         ^.      .       . 

It  is  evident,  then,  that  when  fertilization  is  accom- 
plished the  spermatozoon    must    have  traveled    a    dis- 
tance of  about  twenty-four  centimeters,  the  length  o 
the  upper  part  of  the  vagina  being  taken  to  be  about 
S  cm     that  of  the  uterus  as  7  cm.,  and  that  of  the  tube 
as  12  cm      A  considerable  interval  of  time  is  required 
for  the  completion    of  this  journey    even   though    the 
movement  of  the  spermatozoon  be  tolerably  rapid.     The 
observations  of  Henle  and  Hensen  indicate  that  a  sperma- 
tozoon mav  progress  in  a  straight  line  at  about  the  rate  of 
Irom  I  2  to  27  mm.  per  minute,  while  Lott  finds  the  rate 
to  be  as  high  as  3.6  mm.     Assuming  the  rate  of  progress 
to  be  about  2.5  mm.  per  minute,  the  time  required  by 
the  spermatozoon  to  travel  from  the  upper  part  of  the 
vaccina  to  the  upper  part  of  a  Fallopian  tube  will  be  about 
one  and  a  half  hours   (Strassman).     This,  however,  as- 
sumes that  there  are  no  obstacles  in  the  way  of  the  rapid 
progress   of   the   spermatozoon,  which  is  not  the  case 
since  in  the  first  place,  the  irregularities  and  folds  of  the 
lining  membrane  of  the  tube  render  the  path  of  the  sper- 
matozor.n  a  labvrinthine  one,  and,  secondly,  the  action 
of  the  cilia  of  the  epitheUum  of  the  tube  and  uterus  bemg 


THE    FERTILIZATION    OF   THE   OVUM. 


SI 


from  the  ostium  of  the  tube  toward  the  os  uteri,  it  will 
greatly  retard  the  progress;  furthermore,  it  is  presum- 
able that  the  rapidity  of  movement  of  the  spermatozoon 
diminishes  after  a  certain  interval  of  time.  It  seems 
probable,  therefore,  that  fertilization  does  not  occur  for 
some  hours  after  coition,  even  providing  an  ovum  is  in 
the  tube  awaiting  the  approach  of  the  spermatozoon. 

But  this  condition  is  not  necessarily  present,  and  con- 
sequently the  question  of  the  duration  of  the  vitality  of 
the  sperm  cell  becomes  of  importance.  Ahlfeld  has  found 
that,  when  kept  at  a  proper  temperature,  a  spermatozoon 
will  retain  its  vitality  outside  the  body  for  eight  days,  and 
Diihrssen  reports  a  case  in  which  living  spermatozoa  were 
found  in  a  Fallopian  tube  removed  from  a  patient  who  had 
last  been  in  coitu  about  three  and  a  half  weeks  previously. 
As  regards  the  duration  of  the  vitality  of  the  ovum  less 
accurate  data  are  available.  Hyrtl  found  an  apparently 
normal  ovum  in  the  uterine  portion  of  the  left  tube  of  a 
female  who  died  three  days  after  the  occurrence  of  her 
second  menstruation,  and  Issmer  estimates  the  duration 
of  the  capacity  for  fertilization  of  an  ovum  to  be  about 
sixteen  days. 

It  is  evident,  then,  that  even  when  the  exact  date  of  the 
coitus  which  led  to  the  fertilization  is  known,  the  actual 
iiioment  of  the  latter  process  can  only  be  approximated, 
and  in  the  immense  majority  of  cases  it  is  necessary  to 
rely  upon  the  date  of  the  la^t  menstruation  for  an  estima- 
tion of  the  probable  date  of  parturition.  And  by  this 
method  the  possibilities  for  error  are  much  greater.  It 
lias  been  seen  that  ovulation  usually,  though  not  invari- 
ably, is  associated  with  menstruation,  but  it  is  uncertain 
whether  the  ovum  whose  fertilization  has  resulted  in  a 
pregnancy  was  expelled  from  its  follicle  during  the  last 
menstrual  period  which  occurred,  or  during  or  just  pre- 


I  i. 


52 


THE    DEVELOPMENT    OF    THE    HUMAN    BODY. 


ceding  the  first  omitted  period.  Both  views  have  been 
advocated,  but  it  seems  probable  that  the  latter  case  is 
the  more  frequent,  the  fertilized  ovum  being  one  which 
has  been  expelled  from  its  follicle  subsequent  to  the  last 
menstruation  which  occurred.  The  duration  of  preg- 
nancy is  normally  ten  lunar  or  about  nine  calendar  months 
and  it  is  customary  to  estimate  the  probable  date  of  par- 
turition as  nine  mr"ths  and  seven  days  from  the  last 
menstruation.  From  what  has  be  ^n  said,  it  is  clear  that 
any  such  estimation  can  be  depended  upon  only  as  an 
approximation,  the  possible  variation  from  it  being  con- 
siderable. 

Superfetation. — The  occasional  occurrence  of  twin  fetuses 
in  different  stages  of  development  has  suggested  the  possi- 
bility of  the  fertilization  of  a  second  ovum  as  the  result  of  a 
coition  at  an  appreciable  interval  of  time  after  the  first  ovum 
has  started  upon  its  development.  There  seems  to  be  little 
room  for  doubt  but  that  many  of  the  cases  of  supposed  super- 
fetation,  as  this  phenomenon  is  termed,  are  instances  of  the 
simultaneous  fertilization  of  two  ova,  one  of  which,  for  some 
cause  concerned  with  the  supply  of  nutrition,  has  later  failed 
to  develop  as  rapidly  as  the  other.  At  the  same  time,  how- 
ever, even  although  the  phenomenon  may  be  of  rare  occiurence, 
it  is  by  no  means  impossible,  for  occasionally  a  second  Graafian 
follicle,  either  in  the  same  or  the  other  ovar>',  may  be  so  near 
maturity  that  its  ovum  is  extruded  soon  after  the  first  one, 
and  if  the  development  of  the  latter  and  the  incidental  changes 
in  the  uterine  mucous  membrane  have  not  proceeded  so  far 
as  to  pr-'vent  the  access  of  the  spermatozoon  to  the  ovum, 
its  fertilization  and  development  may  ensue.  The  changes, 
however,  which  prevent  the  passage  of  the  spermatozoon  are 
completed  early  in  development  and  the  differences  between 
the  normally  developed  embr>o  and  that  due  to  superfetation 
will  be  comparatively  small,  and  will  become  less  and  less 
evident  as  development  proceeds,  provided  that  the  supply  of 
nutrition  to  both  embryos  is  equal. 


LITERATURE. 


53 


LITERATURE, 
E.  Ballowitz:  "  Untersuchungen  iiber  die  Struktur  der  Spcr-natozoen," 

No.  4.     Zeitschr.  jiir  wisscnscli.  ZooL,  Lii,  1891. 
K.  VON  BardelEben:  "Beitriige  zur  Histologic  des  Hodens  und  zur 
Spermatogenese   beim   Menschen,"    Arcliiv  jur  Anaf.  und  Physiol., 
Aruit.  Abth.,  Supplement,  1897. 
Th.   BovERI:   " Befruchtung,"   Ergcbnisse  der  Anal,   und  Entuuklungs- 

gcsch.,  I,  1892. 
J.  G.  Clark:  "Ursprung,  Wachstliuiu  und  Kiule  des  Corpus  luteuni  nach 
Beobacbtungen   am    Ovarium    des   Schweincs  und   des   Menschen," 
Archiv  jur  Anat.  und  Physiol.,  Anat.  Abth.,  1898. 
\V.  HeapE:  "The  Menstruation  of  Semnopithecus  entellus,"  J'hilu.soph 

Trans.  Royal  Soc,  cuxxxv.  1894. 
W.  Heape:  "The  Menstruation  and  Ovulation  of  Macacus  rhesus  with 
Observations  on  the  Changes  Undergone  by  the  Discharged  Follicle," 
Philosoph.  Trans.  Royal  Soc,  CLXXXViii,  1897 
().   Hertwig:   "Vergleich  der  Ei-  und  Samenbildung  bei  Nematoden," 
Archiv  fiir  mikrosk.  Anat.,  xxxvi,   1890. 

hossEk:   "Untersuchungen  iiber  Sp- rmatogencse,"    Archiv 

I.  Anat.,  LI,   1898. 
jeber  Struktur  und  Histogenese  der  Samenfiiden  des  Meer- 
iiens,"  Archiv  jiir  mikrosk.  Anat.,  iiv,  1899. 
K.  S.  Moore:  "Some  Points  in  the  Sperm? togenesis  of  Mammalia," 
Internal.  Monatsschrijt  jar  Anat.  und  Physiol.,  xi,   1894. 
\V.  Nagel:  "Das  menschliche  Ei,"  Archiv  jUr  mikrosk.  Anat.,  xxxi,  1888. 
G.  NiESSiNC. :  "  Die  Betheiligung  der  Centralkorper  und  Sphare  am  Aufbau 
des   Samenfadens    bei    Siiugethieren,"    Archiv    jiir   mikrosk.    Anat., 
XLViii,  1896. 
J.  Sobotta:  "  Die  Befruchtung  und  Eurchung  des  Eics  der  Maus,"  Archiv 

jiir  mikrosk.  Anat.,  XLV,  1895. 
J.  Sobotta:  "Ueber  die  Bildung  des  Corpus  luteum  bei  der  Maus,"  Archiv 
jiir  mikrosk.  Anat.,  XLVII,   1897. 
Sobotta:  "I'eber  die  Bildung  des  Corpus  luteum  beini  Kaninchen," 

Anat.  llejtc,  viii,  1897. 
Strassmann:  "Beitriige  zur  Lehre  von  der  Ovulation,  Menstruation 
wnA  Conception,"   Archiv  jiir  GynaekoL,  Lii,    1896. 
\V.  W.xLnKVER:  "Eierstock  und  Ei,"  Leipzig,  1870. 


M.    VON 

ji 
E.  ME 

sc. 

J 


J 


1 


CHAPTER  II. 

THE  SEGME^  TATION  OF  THE  OVUM  AND  THE 
FORMATION  OF  THE  GERM  LAYERS. 

Segmentation.— The  union  of  the  male  and  female  pro- 
nuclei has  already  been  described  as  being  accompanied 
by  the  formation  of  a  mitotic  spindle  which  produces  a 
division  of  the  ovum  into  two  cells  This  first  division  is 
succeeded  at  more  or  less  regular  intervals  by  others  until 
a  mass  of  cells  is  produced  in  which  a  differentiation 
eventually  appears.  These  divisions  of  the  ovum  con- 
stitute what  is  termed  its  segmentation. 

The  mammalian  ovum  has  behind  it  a  ]  ^ng  line  of  evo- 
lution, and  even  at  early  stages  in  its  development  it 
exhibits  peculiarities  which  can  only  be  reasonably  ex- 
plained as  an  inheritance  of  past  conditions.  One  of  the 
most  potent  factors  in  modifying  the  character  of  the  seg- 
mentation of  the  ovum  is  the  amount  of  food  yolk  which 
it  contains,  and  it  seems  to  be  certain  that  the  immediate 
ancestors  of  the  mammalia  were  forms  whose  ova  con- 
tained a  considerable  amount  of  yolk,  many  of  the  pecu- 
liarities resulting  from  its  presence  being  still  clearly 
indicated  in  the  early  development  of  the  almost  yolkless 
mammalian  ovum .  To  give  some  idea  of  the  peculiarities 
which  result  from  the  presence  of  considerable  amounts  of 
yolk  it  will  be  well  to  compare  the  processes  of  segmenta- 
tion and  differentiation  seen  in  ova  with  different  amounts 
of  yolk. 

A  little  below  the  scale  of  the  vertebrates  proper  is  a 
form,  Amphioxus,  which  possesses  an  almost  yolkless 

54 


SEGMENTATION    OF   THE   OVUM. 


55 


ovum  presenting  a  simple  process  of  development.  The 
fertilized  ovum  of  Amphioxus  in  its  first  division  separates 
into  two  similar  and  equal  cells,  and  these  are  made  four 
(Fig.  1 6,  A)  bv  a  second  plane  of  division  which  cuts  the 
previous  one  at  right  angles.  A  third  plane  at  right  angles 
to  both  the  preceding  ones  brings  about  an  eight-celled 
stage  (Fig.  i6,  B),  and  further  divisions  result  in  the  for- 
mation of  a  large  number  of  cells  which  arrange  them- 


~3 

=3-- 


Mh  Fio.  16.— Stages  in  the  SeomEnt.\Tion  ok  Amphioxus. 

Mr  A,  Four-celled  stage;  H,  eight-celled  stage;  C,  sixteen-celled  stage;  D, 

^k  early  blastula;  E,  blastula;  I\  section  of  hlastula.  -(//a^jc/ic*.) 

^  selves  in  the  form  of  a  hollow  sphere  which  is  known  as  a 

blastula  (Fig.  i6,  K). 

The  minute  amount  of  yolk  which  is  present  in  the 

'M  ovum  of  Amphioxus  collects  at  an  early  stage  of  the  seg- 

%  mentation  at  one  pole  of  the  ovum,  the  cells  containing  it 

'  being  somewhat  larger  than  those  of  the  other  pole  (F'ig. 

1 6,  B),  and  in  the  blastula  the  cells  of  one  pole  are  larger 

^  and  more  richly  laden  with  yolk  than  those  of  the  other 

pole  (Fig.  1 6,  F).     If,  now,  the  segmenting  ovum  of  an 

Amphibian  be  examined,  it  will  be  found  that  a  very 


56 


THE  DEVELOPMENT  OF  THE  HUMAN  BODY. 


much  greater  amount  of  yolk  is  present  and,  as  in  ^w- 
phioxus,  it  is  located  especially  at  one  pole  of  the  ovum. 
The  first  three  planes  of  segmentation  have  the  same 
relative  positions  as  in  Amphioxus  (Fig.  16),  but  one  of 
the  tiers  of  cells  of  the  eight-celled  stage  is  very  much 
smaller  than  the  other  (Fig.  17,  B).     In  the  subsequent 


B 


C  D 

Fig.   17.     Stacks   in  tmk    Segmentation   of   Amhlystoma. 

Iiymcr.) 


-(Eyclcs- 


stages  of  segmentation  the  small  cells  of  the  upper  pole 
divide  more  rapidly  than  the  larger  ones  of  the  lower  pole, 
the  activity  of  the  latter  seeming  to  be  retarded  by  the 
accumulation  of  the  yolk,  and  the  resulting  blastula  (Fig. 
17,  D)  shows  a  very  decided  difference  in  the  size  of  the 
cells  of  the  two  poles. 


M 

vs. 


SEGMENTATION    OF   THE    OVUM. 


57 


In  the  ova  of  reptiles  and  birds  the  amount  of  yolk 
stored  up  in  the  ovum  is  very  much  greater  even  than  in 
the  amphibia,  and  it  is  aggregated  at  one  pole  of  the  ovum 
of  which  it  forms  the  principal  mass,  the  yolkless  proto- 
plasm appearing  as  a  small  disk  upon  the  surface  of  a 
relatively  huge  mass  of  yolk.     The  inertia  of  this  mass  of 


Fic.  18.     FoiR  Stacks  in  the  Sec.mentation  ok  the  Blastoderm 
OF  the  Chick.— (CW/c.) 


nutritive  material  is  so  great  that  the  segmentation  is 
confined  to  the  small  yolkless  disk  of  protoplasm  and 
affects  consequently  only  a  portion  of  the  entire  ovum. 
To  distinguish  this  form  of  segmentation  from  that  which 
affects  the  entire  ovum  it  is  termed  meroblastic  segmenta- 
tion, the  othe  form  being  known  as  holohlastic. 
5 


r 


• 


58 


THE    DEVELOPMENT    OF    THE    HUMAN    BODY. 


In  the  ovum  of  a  turtle  or  a  bird  the  first  plane  of  seg- 
mentation crosses  the  protoplasmic  disk,  dividmg  it  mto 
two  practically  equal  halves,  and  the  second  plane  forms 
at  approximately  right  angles  to  the  first  one  dividmg 
the  disk  into  four  quadrants  (Fig.  18,  A).  The  third 
division,  like  the  two  which  precede  it,  is  radial  m  position 
while  the  fourth  is  circular  and  cuts  off  the  in^^r  ends  of 
the  six  cells  previously  formed  (Fig.  18.  D).  Jh^  <iisk 
now  consists  of  six  central  smaller  cells  surrounded  by  six 


if 


Amphibian. 

6/,  Blastoderm ;  >',  yolk-mass. 

larger  peripheral  ones.  Beyond  this  period  no  regularity 
can  be  discerned  in  the  appearance  of  the  segmentation 
planes-  but  radial  and  circular  divisions  continuing  to 
form  the  disk  becomes  divided  into  a  large  number  of 
cells 'those  at  the  center  being  much  smaller  than  those 
at  the  peripherv.  In  the  mean  time,  however  the 
smaller  central  cells  have  begun  to  divide  in  planes 
parallel  to  the  surface  of  the  disk,  which,  from  being  a 
simple  plate  of  cells,  thus  becomes  a  discoidal  cell-mass. 


SEGMENTATION    OF    THE   OVUM. 


59 


During  the  segmentation  of  the  disk  it  has  increased 
materially  in  size,  extending  further  and  further  over  the 
surface  of  the  yolk,  into  the  substance  of  which  some  of 
the  lower  cells  of  the  discoidal  cell- mass  have  penetrated. 
A  comparison  of  the  diagram  (Fig.  19)  of  the  ovum  of 
a  reptile  at  about  this  stage  of  development  with  the 
figure  of  the  amphibian  blastula  (Fig.  17,  D)  will  indicate 
the  similarity  between  the  two,  the  large  yolk-mass  of  the 
reptile  (Y)  with  the  scattered  cells  which  it  contains  cor- 
responding  to  the  lower  pole  cells  of  the  amphibian  blas- 
tula the  central  cavity  of  which  is  practically  suppressed 
in  the  reptile.  Beyond  this  stage,  however,  the  similarity 
becomes  more  obscured.  The  peripheral  cells  of  the  disk 
continue  to  extend  over  the  surface  of  the  yolk  and  finally 
completely  enclose  it,  forming  an  enveloping  layer  which 
lis  completed  at  the  upper  pole  of  the  egg  by  the  discoidal 
*  cell-mass,  or,  as  it  is  usually  termed,  the  blastoderm. 

Turning  now  to  the  mammalia,*  it  will  be  found  that 
the  ovum  in  the  great  majority  is  almost  or  quite  as  desti- 
tute of  food  yolk  as  is  the  ovum  of  Amphioxus,  with  the 
result  that  the  segmentation  is  of  the  total  or  holoblastic 
type.  It  does  not,  however,  proceed  with  that  regularity 
wliich  marks  the  segmentation  of  A  mphioxus  or  an  amphi- 
bian, but  while  at  first  it  divides  into  two  slightly  unequal 
cells  (Fig.  20),  thereafter  the  divisions  become  irregular, 
three-celled,  four-celled,  five-celled,  and  six-celled  stages 
having  been  observed  in  various  instances.  Nor  is  the 
result  of  the  final  segmentation  a  hollow  vesicle  or  blas- 
tula, but  a  solid  mass  of  cells,  termed  a  morula,  is  formed. 
This  structure  is  not,  however,  comparable  to  the  blastula 
of  the  lower  forms,  but  corresponds  to  a  stage  of  reptilian 


*  The  segmentation  of  the  human  ovum  has  not  yet  been  observed ; 
wliat  follows  is  based  on  what  occurs  in  the  ovum  of  the  rabbit,  mole, 
and  especially  of  a  bat  (Van  Beneden). 


6o 


TMK    IIEVKLOI'MKNT    OF     THE    HUMAN    IIOHV. 


development  a  little  later  than  that  shown  in  Fig.  19, 
since,  as  will  be  shown  directly,  the  cells  corresponding  to 
the  blastoderm  and  the  enveloping  layer  are  already 
present.  There  is,  then,  no  blastulajit_age  in  the  mamma- 
lian development. 


U 


Fig.  20. — FotR   St.\i.i;s  in   tiik  Skumkntation  ok   tiik  Ovcm  of  a 

M(M'SK. 

X,  I'olar  j;l()l)ule.      {Subotta). 


This  differentiation  now  begins  by  the  peripheral  cells 
of  the  morula  becoming  less  spherical  in  shape  and  later 
forming  a  layer  of  flattened  cells,  the  enveloping  layer, 
surrounding  the  more  spherical  central  cells  (Fig.  21,  A). 
In  the  latter  vacuoles  now  make  their  appearance,  espe- 


SECMENIATION    OF   THE   OVUM. 


6i 


B 


/. 


"^z. 


^^^^o-  ¥ 


..^.u^"^' 


Fin.  2!. 


c  n 

— Later  Stages  in  the  Secmentxtion  ok  the  Ovim  ok  a 

Bat. 
C,  and  I)  are  sections,  W  a  surface  view. — {Van  liencden.) 


62 


TlIK    DKVKI.OJ'M.  ^  C    iM 


1  111 


I  \  \N  nonv. 


11 


It 


cially  in  lliost-  cells  wiii' Ii  i-i,  ccartst  what  may  be 
rejjarded  as  the  lower  pole  of  '!:  ovum  (Fig.  2i,  C), 
and  these  \-acuoks,  gradual!  v  iiai «  i'  ug  in  si/e, eventually 
become  confluent,  the  tondition  repn  rented  in  Fig.  21,  I), 
being  produced.  At  tlii^  stage  the  (j\  um  ccmsists  of  an 
envxlupiag  layer,  endosjiij;  _a  cavity  vyhicli  is  equivalent 
to  the  yolk-mass  of  the  reptilian  ovum,  the  vacuolization 
of  the  inner  cells  of  the  morula  representing  a  belated 
formation,  of  yolk.  On  the  inner  surface  of  the  envelop- 
ing layer,  at  what  may  be  termed  the  upper  pole  of  the 
ovum,  is  a  mass  of  cells  pmjecting  into  the  yoUucavity 
and  forming  what  is  known  as  the  inner  cell-mass,  a 
structure  comparable  to  the  blastoderm  of  the  reptile. 
In  one  respect,  however,  a  difference  ,jbtains,  the  inner 
cell-mass  being  completely  enclosed  within  the  enveloping 
cells,  which  is  not  tin  case  with  tlie  blastoderm  of  the  rep- 
tile. That  portion  of  the  enveloping  layer  which  covers 
the  cell-mass  has  been  termed  Roubir's  covering  layer,  and 
probably  owes  its  existence  to  the  precocity  of  the  forma- 
tion of  the  enveloping  layer. 

It  is  clear,  then,  that  an  explanation  of  tlie  (  arly  stages 
of  development  of  the  mammalian  ovum  is  to  he  obtained 
by  a  comparison,  not  with  a  yolkless  ovum  sum  as  that  of 
Amphioxus,  but  with  an  ovum  richly  laden  with  yolk, 
such  as  the  meroblastij;  o       n  of  a  reptile  or  bird.     In 
these  forms  the  nutrition  necessary  for  the  growth  of  the 
embryo  and  for  the  compluated  processes  of  development 
is  provided  for  by  the  stonn.^  up  of  a  quantit .  of  volk  in 
the  ovum,  the  embryo  being  thu-  independe?     of  e:    vmA 
sources  for  food.     The  same  is  true  also  o    the  1«  we 
mammalia,  the  Monotrt=mer.,  which  are  egg-l   ving  fern-: 
producing  ova   resembling  greatly   those    oi    a    reptil< 
When,  however,  in  the  higher  mammals  the  nutrition    A 
the  embryo  became  provide<l  for  by  t  .se  attachmen'    d  tfoe 


'^ 


THE    SEGMENTATluN    OF 


II  i--.    OVUM. 


«3 


..mbrvo  to  th.-  walh  ..r  tbe  u.«uH  ..f  U,.  paront  so  that  it 

JuX  nour,4,e.l  directl,  .v  tu,    .arc-n,,  the  s  on„«  up 

,     \l  in  ll„   ovum  was  .    iicc.      iry  an.i  it  Warn,   a 

l'„;t.  "  o  u,;  alth.mgh  .na„>  ,    culi.    tics  depend,    , 

!  *K  oriKUK,:  mcr„l,l..stic  .  •  uditic.n  per,    te<.  .n  Us  de- 

^         n         h,     h/  -  \s  a  rule,  in  the  hunuui  species  ».ut  ou,- 
Twin  0«^"  '/":;«'     :^^^  ,,„t  ,he  mcurrence  of  twins  .s  bv 
.n.brvo  .kvilopsat  a  ^""\'  ?";  '"     ^j  ^^,^,,^  quadruplets  occa 

fT"^'  uLTtLrfothrU[taneousrirK.ningandfer    Uza- 
to  two     uises,  *  '"^^11'"^  ,„  fj-oni  both    n-anes,  oi  .     the 

separation  of  a  MiWt  i  development.       ha'    twir 

r„^^.eToLced  S-^^^  "lauer  ■.  ^ocess  ,L  been     .undant 
Sn  bv  exper.mentatiot.  upon  d-    elopinR  ova  of  lo 
^h  of  U.e  U.O  cells  ol  a     An^^^   oxus  ovum  in  th. 
"elopm-^t,   il   mechanically    -P'^rated    comp 
velopmern  and  producing  an  embrvo  of  au  ut  h 

,ccurrence    .f  d<  able  monsters  is  explame  an  ;^"P«rt*f^ 

i;aS"m  mt.     .o  parts    .f  an  ong;-nv^.n^  embryo    U^ 
extent  of  the  ..,<aration,  and  probaTv     'Iso       e  ^tage J^^^ 

■2r:r,he  .::stdiv.:x^o^rs.g^\he^;ifr\ii 

-XioL  .>f  separation  occar,/-,^  -     -'^^^^^^^ 


tage 

4   it     dt 
e  n     ma 


o  forms  in 
u.  the  entire 
iso  affect  onh 
:a'nce,  doubli 
ms  r.f  so-called 
nly  a  group  of 


tion  as  si  in  such  cases  as  the  Siam. 
which  the  vvt.  individu:  s  are  united 
len-nh  of  heir  b(  dies.  he  ^cparatm; 
a  portion  if  le  embry,  i^ioducmg, 
laced  or  dt  able  headt  i  ni  nsterj,  .  various  h 
nanisitic  re  )nsters;  and,  finally,  n  rtiay  affect  ..,  „ 
cells  destined  tt  form  a  special  organ,  producmg  an  excess  m 
narts  such  as  supernumerar>'  digits  or  accessor>-  spleens. 
^  t  iiarbeen  obser^  .d  in  tlie  case  .'  U.uble  monsters  that  one 
„i  the  two  .  sed  idividuals  ahNa  '  s  the  position  of  its 
various  --rgans  re  rsed,  it  being  .^  i  were,  the  lookin-- 
dass  imasre  of  it  fellow.  Cases  of  a  s  milar  .itus  inversus 
t^Z:%  it  is  ..lied,  have  not  infrequently  been  observed 
in  single  individuals,  and  a  plausible  explanation  "f  ^u^j^ J^J^J 
regards  them  as  one  of  a  pair  of  twins  formed  by  the  division 


64 


THE    DEVELOPMENT    OF   THE    HUMAN    BODY. 


!    I 


'  i 


of  a  single  embryo,  the  other  individual  having  ceased  to 
develop  and  either  having  undergone  degeneration  or,  if  the 
separation  was  an  incomplete  one,  being  included  within  the 
body  of  the  apparently  single  individual. 

The  Formation  of  the  Germ  Layers.— During  the  stages 
which  have  been  described  as  belonging  to  the  segmenta- 
tion period  of  development  there  has  been  but  little  differ- 
entiation of  the  cells.  In  Amphioxus  and  the  amphibians 
the  cells  at  one  pole  of  the  blastula  are  larger  and  more 
yolk-laden  than  those  at  the  other  pole,  and  in  the  mam- 
mals an  inner  cell-mass  can  be  distinguished  from  the 
enveloping  cells,  this  latter  differentiation  having  been 
anticipated  in  the  reptiles  and  being  a  differentiation  of  a 
portion  of  the  ovum  from  which  alone  the  embryo  will 
develop  from  a  portion  which  will  give  rise  to  accessory 
structures.  In  later  stages  a  differentiation  of  the  inner 
cell-mass  occurs,  resulting  first  of  all  in  the  formation  of  a 
two-layered  or  diplohlastic  and  later  of  a  three-layered  or 
triplohlastic  stage. 

Just  as  the  segmentation  has  been  shown  to  be  pro- 
foundly modified  by  the  amount  of  yolk  present  in  the 
ovum  and  by  its  secondary  reduction,  so,  too,  the  forma- 
tion of  the  three  primitive  layers  is  much  modified  by  the 
same  cause,  and  to  get  a  clear  understanding  of  the  forma- 
tion of  the  triploblastic  condition  of  the  mammal  it  will  be 
necessary  to  describe  briefly  its  development  in  lower 
forms. 

In  Amphioxus  the  diploblastic  condition  results  from 
the  flattening  of  the  large-celled  pole  of  the  blastula  (Fig. 
2  2,  A),  and  finally  from  the  invagination  of  this  portion  of 
the  vesicle  within  the  other  portion  (Fig.  22,  B).  The 
original  single-walled  blastula  in  this  way  becomes  con- 
verted into  a  double-walled  sac  termed  a  gastrula,  the 
outer  layer  of  which  is  known  as  the  ectoderm  or  epiblast 


-I 


\ 


X 


THE    FORMATION    OF    THE   GERM    LAYERS. 


65 


and  the  inner  layer  as  the  endoderm  or  hypoblast.  The 
cavity  bounded  by  the  endoderm  is  the  primitive  gut  or 
archenteron,  and  the  opening  by  which  this  communicates 
with  the  exterior  is  the  blastopore.  This  last  structure  is 
at  first  a  very  wide  opening,  but  as  development  proceeds 
it  becomes  smaller,  and  finally  is  a  relatively  small  opening 
situated  at  th-    posterior  extremity  of  what  will  be  the 

.  dorsal  surface  of  the  embryo.  .    •     •.    ,  ..^ 

As  the  oval  embryo  continues  to  elongate  in  its  later 

development  the  third  layer  or  mesoderm  makes  its  ap- 


A  ^ 

Fir,    22  -Two  Staoes  m  the  Gastruuation  op  Amphtoxus.— {Morgan 

and  Hazen.) 

pearance.  It  arises  as  a  lateral  fold  (mp)  of  the  dorsal 
surface  of  the  endoderm  (en)  on  each  side  of  the  middle 
line  as  indicated  in  the  transverse  section  shown  in  Fig.  23. 
This  fold  eventually  becomes  completely  constricted  off 
from  the  endoderm  and  forms  a  hollow  plate  occupying 
the  space  between  the  ectoderm  and  endoderm,  the  cavity 
which  it  contains  being  the  body-cavity  or  coeloni. 

In  the  amphibia,  where  the  amount  of  yolk  is  very 
much  greater  than  in  Amphioxus,  the  gastrulation  be- 
comes considerably  modified .    On  the  line  where  the  large- 


X 


\ 


' 


I    1 

I 

'         'i 

'         i 

.1         ' 

66 


THE  DEVELOPMENT  OF  THE  HUMAN  BODY. 


rmp 


and  small-celled  portions  of  the  blastula  become  con- 
tinuous a  crescentic  groove  appears  and  deepening  forms 
an  invagination  (Fig.  24,  gc)  the  roof  of  which  is  composed 
of  relatively  small  yolk-containing  cells  while  its  floor  is 
formed  by  the  larje  cells  of  the  lower  pole  of  the  blastula. 
The  cavity  of  the  blastula  is  not  sufficiently  large  to  allow 
of  the  typical  invagination  of  all  these  large  cells,  so  that 
they  become  enclosed  by  the  rapid  growth  of  the  ectoderm 
cells  of  the  upper  pole  of  the  ovum  over  them.     Before 

this  growth  takes  place  the 
blastopore  corresponds  to 
the  entire  area  occupied  by 
the  large  yolk  cells,  but 
later,  as  the  growth  of  the 
smaller  cells  gradually  en- 
closes the  larger  ones,  it 
becomes  smaller  and  is 
finally  represented  by  a 
small  opening  situated  at 
what  will  be  the  hind  end 
of  the  embryo. 

toon  after  the  archen- 
teron  has  been  formed  a 
solid  plate  of  cells,  eventu- 
ally splitting  into  two  lay- 
ers, arises  from  its  roof  on 
each  side  of  the  median  line  and  grows  out  with  the 
space  between  the  ectoderm  and  endoderm  (Fig.  25, 
tnk^  and  mk^)  evidently  corresponding  to  the  hollow 
plates  formed  in  the  same  situations  in  Amphioxus. 
This  is  not,  however,  the  only  source  of  the  mesoderm  in 
the  amphibia,  for  while  the  blastopore  is  still  fjuite  large 
there  may  be  found  surrounding  it  between  the  endoderm 
and  ectoderm  a  ring  of  mesodermal  tissue  (Fig.  24,  mes). 


Fk;.  2.V  Transverse  vSection  of 
.\inf>liio\u\  Hmbryo  with  Five 
Mksodermic  Poi'ches. 

tVi,  Notochord;  </,  digestive  cavity; 
i'i\  ectoderm ;  in,  endoderm ;  m,  me- 
dullary plate;  m/>,  mesodermic 
pouch.-    (Halsclick.) 


THE    FORMATION    OF   THE   GERM    LAYERS. 


67 


As  the  blastopore  diminishes  in  size  and  its  lips  come 
together  and  unite,  the  ring  of  mesoderm  forms  first  an 
oval  and  then  a  band  lying  beneath  the  line  of  closure  of 
the  blastopore  and  united  with  both  the  superjacent 
ectoderm  and  the  subjacent  endoderm.  This  Ime  of 
fusion  of  the  three  germ  layers  is  known  as  the  pnmxhve 
streak      It  is  convenient  to  distinguish  the  mesoderm  of 


•s 


mes 


mes. 


Fic.  24-  Section  through  a  Gastrula  of  Amhlystoma. 
Idl,  Dorsal  lip  of  blastopore;  ^c,  digestive  cavity;  gr,  area  of   mesoderm 
I  formation;  mes,   mesoderm.— '(Eycleshymer.) 

I  the  primitive  streak  from  that  formed  from  the  dorsal 
wall  of  the  archenteron  by  speaking  of  the  former  as  the 
Iprostomial  and  the  latter  as  the  gastral  mesoderm,  though 
[  it  must  be  understood  that  the  two  are  continuous  imme- 
diately in  front  of  the  definitive  blastopore. 

In  the  reptilia  still  greater  modifications  are  found  in 
the  method  of  formation  of  the  germ  layers.  Before  the 
enveloping  cells  have  completely  surrounded  the  yolk- 


4 


%t 


.ill 


68 


THE    nEVEI.OPMENT    OF    THE    HUMAN    BODV. 


mass,  a  crescentic  groove,  resembling  that  occurring  in 
amphibia,  appears  near  the  posterior  edge  of  the  blasto- 
derm, the  cells  of  which,  m  front  of  the  groove,  arrange 
themselves  in  a  superficial  layer  one  cell  thick  which  may 
be  regarded  as  the  ectoderm  (Vig.  26,  ec)  and  a  subjacent 
mass  of  somewhat  scattered  cells.  Later  the  lowermost 
cells  of  this  subjacent  mass  arrange  themselves  in  a  con- 
tinuous layer  constituting  what  is  termed  the  primarv 


Fig.  25.  Section  through  an  Kmbrvo  Amphibian  (Triton)  of  24 
Days,  showing  the  Formation  of  the  r, astral  Mesoderm. 

ak,  F,c((»flerm  ;  rli,  rliorda  cndoderin  ,  itk,  digestive  cavity;  ik,  endo- 
derin,  mk^  ;ind  mh',  .splanclinic  and  s(jin:itic  layers  of  the'  meso- 
derm     />,  dorsal  and  l,  ventral      {//irtwig.} 

endoderm  (en^),  while  the  reni;iiiung  colls,  aggregated 
especially  in  the  region  of  the  crescentic  groovf^,  form  the 
prostomial  mesoderm  (prm).  In  the  region  enclosed  by 
the  groove  a  distinct  delimitation  of  the  various  layers 
does  not  occur,  and  this  region  forms  the  primitive  streak. 
The  groove  now  begins  to  deepen,  forming  an  invagination 
of  secondary  endoderm.  the  intent  of  this  invagination 
li*rs??,  lir>wever,  very  if!«t»rent  in  different  rpeeies.  In 
the  gecko  (Will^  it  poshes  forward  between  the  ectoderm 
and  primary  endoderm  almost  to  the  anterior  edge  of  the 


V 


THE    FORMATION    OF    GERM    LAYERS. 


69 


blastoderm,  but  later  the  cells  forming  its  floor,  together 
with  those  of  the  primary  endoderm  immediately  below, 
undergo  a  degeneration,  the  roof  cells  at  the  lateral  mar- 
gins of  the  invagination  becoming  continuous  with  the 
persisting  portions  of  the  primary  endoderm.  This 
layer,  following  the  enveloping  cells  in  their  growth  over 
the  yolk-mass,  gradually  surrounds  that  structure  so  that 


ri^f-l-^Z-l^. 


»'/»  •55 


en. 


prm      ,  St-.' 


:~'-^>rr.ee 


en  C 


ee 


<* 


Fig,  26.     Longitudinal  Sections  throi'(;h  Kmbryos  of  the  Gecko, 

SHOWING    GaSTKUUATION. 

ec.    Ectoderm;   en,  secondary   «ndoderni;  en',  primary    endoderm;  prm, 
prostomi J .  mesoderm . — (I I  i//. ) 

it  comes  to  lie  within  the  archenteron.  In  some  turtles, 
on  the  other  hand,  the  disappearance  of  the  floor  of  the 
invagination  takes  place  at  a  very  early  stage  of  the 
infolding,  the  roof  cells  only  persisting  to  grow  forward  to 
form  the  dorsal  wall  of  the  archenteron.  This  interest- 
ing abbreviation  of  the  process  occurring  in  the  gecko 
indicates  the  mode  of  development  which  is  found  in  the 
mammalia. 


I 

i 


[.: 


H 


70 


HIE    DKVELOPMENT    OF    THE    HUMAN    BODY, 


The  existence  of  a  prostomial  niesodenn  in  connection 
with  the  primitive  streak  has  already  been  noted,  and 
when  the  invagination  takes  place  it  is  carried  forward  as 
a  narrow  band  of  cells  on  each  side  of  the  sac  of  secondary 
endoderm.  After  the  absorption  of  the  ventral  wall  of 
the  invagination  a  folding  or  turning  in  of  the  margins  of 
the  secondary  endoderm  occurs   (Fig.   27)   whereby  its 


Fk;.  27.— Diagrams   iLHsTRATixf;    the    Formation   of   the   Gastral 

Mesoderm  i.v  the  Gecko. 

C€,    Chorda     endoderm;   cc,    ectoderm;    en,     secondary   endoderm;   e«', 

primary  endoderm  ;  gw,  ^astral  mesoderm. — (Will.) 

lumen  becomes  reduced  in  size  and  it  passes  off  on  each 
side  into  a  double  plate  of  cells  which  constitute  the  gas- 
tral  mesoderm.  Later  these  plates  separate  from  the 
archenteron  as  in  the  lower  forms.  All  the  prostomial 
mesoderm  does  not,  however,  arise  from  the  primitive 
streak  region,  but  a  considerable  amount  also  has  its  origin 
from  the  ectoderm  covering  the  yolk  outside  the  limits 
of  the  blastoderm  proper,  a  mode  of  origin  which  serves 
to  explain  the  phenomena  later  to  be  describedJor  the 
mammalia.  ~" 


\ 


THE    FORMATION    OK    GERM    LAYERS. 


7> 


In  comparison  with  the  amphibians  and  Amphioxus, 
the  reptiUa  present  a  subordination  of  the  process  of  in- 
vagination in  the  formation  of  the  endoderm,  a  primary 
endoderm  making  its  appearance  independently  of  an 
invagination,  and,  in  association  with  this  subordination, 
there  is  an  early  appearance  of  the  primitive  streak,  which, 
from  analogy  with  what  occurs  in  the  amphibia,  may  be 
assumed  to  represent  a  portion  of  the  blastopore  which  is 
closed  from  the  very  beginning. 

Turning  now  to  the  mammalia,  it  will  be  found  that 
these  peculiarities  become  still  more  emphasized.  The 
inner  cell -mass  of  these  forms  corresponds  to  the  blasto^ 
derm  of  the  reptilian  ovum,  and  the  first  difTerentiation 
which  appeal^  in  it  concerns  the  cells  situated^ next  the 
cavity  of  the  vesicle,  thesTcells^unrtrng  to  form  a  distinct 
layer  ivhirh^jTaduallv^xtends  so  as.t,0=legm-a  complete 
Hning[_toj:he  inner  surface  of  the^nvelopiiig  cells  (Fig.  28, 
A).  These  cells  are  endodermal  and  correspond  to  the 
primary  endoderm  of  Ihe  reptiles. 

Before  the  extension  of  the  endoderm  is  completed, 
however,  cavities  begin  to  appear  in  the  cells  constituting 
the  remainder  of  the  inner  mass,  especially  in  those  imme- 
diately beneath  Rauber's  cells  (Fig.  28,  B),  and  these 
cavities  in  time  coalesce  to  form  a  single  large  cavity 
bounded  above  by  cells  of  the  enveloping  layer  and  below 
by  a  thick  plate  of  cells,  the  embryonic  disk  (Fig.  28,  C). 
The  cavity  so  formed  is  the  amamdu.  cavity,  whose  further 
history  will  be  considered  in  a  subsequent  chapter. 

It  may  be  stated  that  this  cavity  varies  greatly  in  its  de- 
velopment in  different  mammals,  being  entirely  absent  in  the 
rabbit  at  this  stage  of  development  and  reaching  an  excessive 
development  in  such  forms  as  the  rat,  mouse,  and  guinea-pig. 
The  condition  here  described  is  that  which  occurs  in  the  bat 
and  the  mole,  and  it  seems  probable,  from  what  occurs  in 
the  youngest  human  embryos  hitherto  observed,  that  the 
processes  in  man  are  closely  similar. 


-r^L 


— ■■-•^~:^*^- 


72  TIIK    DKVEI.OPMKNT   OF    THE    HUMAN    llODY. 

While  these  changes  have  been  taking  place  a  splitting 
of  the  enveloping  layer  has  occurred,  so  that  the  wall  of 


A 


t 


'if' 


*^  f 


§ig0: 


<^"*i^ 


B 


C 
Fii;.  28.     Sections  ok  Ova  ok  a  Hat  sho\vin(;  (.4)  the  Formation  of 
THE  FIndoderm  anu  {H  and  C)  of  the  Amniotic  Cavity. — {Van 
Heneden. ) 

the  ovum  is  now  formed  of  three  layers,  an  outer  one 
which  may  be  termed  the  trophoblast,  a  middle  one  which 


—  ^■iMir— —    -^--' 


^-  X 


THE    FORMATION    OF    THE    CKKM    LAYERS. 


73 


probably  is  transformed  into  the  extraembryonic  meso- 
derm of  later  stages,  though  its  significance  is  at  present 
somewhat  obscure,  and  an  inner  one  which  is  the  primary 
endoderm.  In  the  bat,  of  whose  ovum  Fig.  28,  C,  repre- 
sents a  section,  that  portion  of  the  middle  layer  which 
forms  the  roof  of  the  amniotic  cavity  disappears,  only  the 


m^ 


p„.  29  —  4  vSiuE  View  of  Ovum  of  Rabbit  Sev  En  Davs  Old  {Kolliker) ; 
'yv,"KMBKYONic  Disk  of  a  Mole  (Heaped ;  C,  Embryonic  Disk  ok  a 
Dof/s  Ovi:m  of  aboit  Fifteen  Days  {Honnct). 
,d,    Knibryonic    disk;    hn,    Hensen's    nude;    mg,   medullary   groove;  /J^, 
primitive  streak;  la,  vascular  area. 

trophoblast  persisting  in  this  region,  but  in  another  form 
t  Mis  is  not  the  case,  the  roof  of  the  cavity  being  composed 
of  botli  the  trophoblast  and  the  middle  layer. 

A  rabbit's  ovum  in  which  there  is  yet  no  amniotic  cavity 
and  no  spHtting  of  the  enveloping  la^^r  shows,  when 
viewed  from  above,  a  relatively  small  dark  area  on  the 
surface,  which  is  the  embrvonic  disk.     But  if  it  be  looked 


I- 


!!i 


{ 

III    • 


74 


THE    DEVEI-OI'MENT    OK    THE    HUMAN    BODV. 


at  from  the  side  (Fig.  29,  A),  it  will  be  seen  that  the  upper 
half  ol  the  ovum,  that  half  in  which  the  embryonic  disk 
occurs,  is  somewhat  darker  than  the  lower  half,  the  line  of 
separation  of  the  two  shades  corresponding  with  the  ed^Q 
of  the  primary  endoderm  which  has  extended  so  far  in  its 
growth  around  the  inner  surface  of  the  enveloping  layer. 
A  little  later  a  dark  area  appears  at  one  end  of  Ihe  em- 
bryonic disk,  produced  by  a  proliferation  of  cells  in  this 
region  and  having  a  somewhat  crescentic  form.  As  the 
embryonic  disk  increases  in  size  a  longitudinal  band  makes 
its  appearance  extending  forward  in  the  median  line 
nearly  to  the  center  of  the  disk  and  represents  the  primi- 


FiG.   30-  I'osTKKioK   Portion  of  a   Longitidinal  Section  throi-cii 

THE    K.MBRYt>NIC    DiSK  OF   A    MoLE. 

bt,    Blastopore;    ..,   ectotlenii;    cu,    endoderm;    f>rm,   prostomial    meso- 
derm.— {After  Ihapc.) 

tive  streuKf  Fi-  29,  B),  a  slight  groove  along  its  median 
line  forming  what  is  termed  the  primitive  groove.  In 
slightly  lat<-r  stages  an  especially  dark  spot  may  be  seen 
at  tlu  front  end  of  the  primitive  streak  and  is  termed 
Hcnseiis  node  (Fig.  29,  C,  /»«),  while  still  later  a  dark 
stn.ik  may  be  observed  extending  forward  from  this  in 
the  median  line  and  is  termed  the  head- process  of  the 
primitive  streak. 

To  understand  the  meaning  of  these  various  dark  areas 
recourse  must  be  had  to  the  studv  of  sections.  A  longi- 
tudinal section  through  the  embryonic  disk  of  a  mole 
ovum  at  the  time  when  the  crescentic  area  makes  its  ap- 


M'^ 


THE    FORMATION    OK    THE    (iERM    LAYERS. 


75 


peurance  is  shown  in  Fig.  30.     Here  there  is  to  be  seen 
near  the  hinder  edge  of  the  disk  what  is  potentially  an 
opening  (hi),  in  front  of  which  the  ectoderm  iec)  and  prim- 
ary endoderm   (en)  can  be  clearly  distinguished,  while 
behind  it  no  such  c   itinction  of  the  two  layers  is  visible. 
This  stage,  then,  may  be  regarded  as  comparable  to,th^ 
invagination  stage  of  the  reptilian  ovum,  the  blastopore 
being,  however,  much  less  developed,  and  the  region  be- 
hind the  blastopore  will  correspond  to  the  reptilian  primi- 
tive streak.     The  later  forward  extension  of  the  primitive 
streak  is  supposed  to  be  due  to  the  mode  of  growth  of  the 
embryonic  disk.     Between  the  stages  represented"  in  Figs. 


'  I  '  ' 
ill 

'   '  '.  ' 
1,1' 

>  1  I  ' 
\  \  \  \ 


^^      \  \ 

^      N     ^        \ 

\    ^     ^     \ 

I  '  '  ' 
I'll 
'  '  '  / 
'      /      /      / 


i 


Fic.  .^l.  -  DiAdRAM  Ilustratinc.  Concrescence.     (,/>«!>!/.) 

30  and  29,  B,  the  disk  has  enlarged  considerably  and  as 
growth  proceeded  there  was  a  turning  in,  as  it  were,  of  the 
edges  of  the  disk  at  its  posterior  end,  whereby  the  primi- 
tive streak  would  be  carried  forward  and  elongated.  This 
process,  which  is  termed  concrcsance,  will  perhaps  be 
understood  more  clearly  from  an  inspection  of  Fig.  31 
than  from  many  lines  of  description.  If  this  process  of 
concrescence  really  occurs,  then  the  point  w  here  the  origi- 
nal rudimentary  blastopore  occurred  is  now  situated  far 
forward  upon  the  embryonic  disk,  and  Hensen's  node 
indicates  a  proliferation  of  cells  in  the  vicinity  of  the 
blastopore  to  form  the  prostomial  mesoderm. 


76 


THE    DKVKI-Ol'MKNT    OF     I  UK    HUMAN    UODV. 


As  regards  the  head  process,  it  is  a  hand  of  cells  which 
grows  forward  from  the  region  of  the  blastopore  along  the 
median  line  and  replaces  the  primary  endodenu  in  that 
situation  (Fig.  32,  chp).  It  corresponds,  therefore,  to 
the  dorsal  wall  of  the  invagination  of  secondary  endoderm 
in  the  reptile,  the  ventral  wall  of  the  invagination  not 
developing  at  all,  a  condensation  of  development  already 
indicated  in  th°  turtle  (see  p.  69).  Indeed,  in  the  gecko, 
the  turtle,  and  the  mammal  we  liave  three  degrees  of  sim 
plification  of  a  process.  In  the  i;ccko  a  sac-like  invagina- 
tion extends  nearly  to  the  anterior  edge  of  the  embryonic 


M 


{•^#' 


Fig.  32.— Transverse  vSectiu.n  ok  the  Hmbrvonic  Area  of  a  Doc's 

Ovum  at  about  the  Sta(;e  of  Deveuoi'me.nt  shown  in  Fu;.  29,  C. 
The   section   passes   tlirouKli    tlie   heart    process  {Chp) ;  -V/,  mesoderm.— 

{liontut.) 


disk  and  its  ventral  wall  later  disappears;  in  the  turtle 
the  invagination  is  comparatively  slight  and  the  useless 
ventral  wall  is  only  partly  developed;  and,  finally,  in  the 
mammal  (Fig.  33)  the  invagination  is  practically  non- 
existent and  no  ventral  wall  whatsoever  is  formed,  only 
the  dorsal  wall  {ce)  growing  forward.  It  should  be  stated 
that  in  some  mammals  apparently  the  most  anterior  por- 
tion of  the  roof  of  the  archenteron  is  formed  directly  from 
the  cells  of  the  primary  endoderm,  which  in  this  region 
are  not  replaced  by  the  head  process,  but  aggregate  to 


t 


TMK    KORMAriON    OF    THK    fJEK  vl    LAj  KRS. 


n 


form  .1  compact  pi  ite  of  cells  with  which  tli.  ant<  rir.r  k\- 
trcmity  t)f  the  head  process  unites.  Such  a  cori'litifm 
would  represent  a  further  modification  oi   the  original 

condition. 

As  rcijards  the  fornuition  of  the  niestMlcrui  ii  is  possible 
to  reco};nize  both  the  prostomial  and  i- astral  nusodirm 
in  th(  mammalian  .vum,  t!.ou,i>h  the  two  parts  are  nol  so 
clearly  di  'inguishable  as  in  lower  forms.  It  has  already 
been  Men  iliat  Hensen's  node  probably  indi«ates  the  ex 
istence  ^"  a  mass  of  prosiomial  niv  oderni  and  when  the 
head  process  grows  forward  it  carries  with  it  some  of  this 


KlO.     33.    -Dl.\     KAM     OK     A     LoNf.ITUDINAU    SECTION    THROUOII    THE     Km- 

BKYONK  Disk  of  a  Mole. 

am.  Amnion;  .,,   chorda  cndnciirin;  cr,  ectoderm ;  wf,  neurenteric  canal 

^f,  primitive  streak.     (Ilrape.) 

tissue.  But,  in  addition  to  this,  a  contribution  to  the 
mesoderm  is  also  apparently  furnished  by  the  cells  of  the 
head  process  in  the  form  of  lateral  plates  situated  on  each 
side  of  the  middle  line.  These  plates  are  at  first  solid 
(Fig. 34,  ^w),but  their  cells  quickly  arrange  themselves  in 
two  layers,  between  which  a  coelomic  space  later  appears. 
Furthermore,  as  has  already  been  pointed  out,  the  layer 
of  enveloping  cells  splits  into  two  concentric  layers,  the 
inner  of  which  seems  to  be  mesodermal  in  its  nature  and 
forms  a  layer  lining  the  interior  of  the  trophoblast  and 
lying  between  this  and  the  primary  endoderm.  This 
layer  is  by  no  means  so  evident  in  the  lower  forms,  but  is 


■liM 


78 


THE    DEVELOPMENT   OF    THE    HUMAN    BODY. 


perhaps  represented  in  the  reptilian  ovum  by  the  cells 
which  underlie  the  ectoderm  in  the  regions  peripheral  to 
the  blastoderm  proper  (see  p.  70). 

The  Significance  of  the  Germ  Layers. — The  formation  of 
the  three  germ  layers  is  a  process  of  fundamental  impor- 
tance, since  it  is  a  differentiation  of  the  cell  units  of  the 
ovum  into  tissues  which  have  definite  tasks  to  fulfil.  As 
has  been  seen,  the  first  stage  in  the  development  of  the 
layers  is  the  formation  of  the  ectoderm  and  endoderm,  or, 
if  the  physiological  nature  of  the  layers  be  considered,  it 
is  the  differentiation  of  a  layer,  the  endoderm,  which  has 


ff/n© 


Vic.  34,     Transverse  Section-  throi  i.ii  the  liMBRvoNic   Disk  of  a 

Kahbit. 
ch,  Cliorda  endofierni ,    .<■,  ectoderm;   cii,  endoderm;  jjw,  gastral    meso- 
derm.    (Ajtrr  Viin  lU-tudrn.) 


principally  nutritive  functions.  In  certain  of  the  lower 
invertebrates,  the  class  Ccelentera.  the  differentiation  does 
not  proceed  beyond  this  diplol)lastic  stage,  but  in  all 
higher  forms  the  intermediate  layer  is  also  developed,  and 
with  its  appearance  a  further  division  of  the  functions  of 
the  organism  supervenes,  the  ectoderm,  situated  upon  the 
outside  of  the  body,  assuming  the  relational  functions,  the 
endoderm  becoming  still  more  exclusively  nutritive,  while 
the  remaining  iurictions,  supportive,  excretory,  loco- 
motor, reproductive,  etc.,  are  assumed  by  tlie  mesoderm. 
The  manifold  adaptations  of  development  obscure  in 


% 


THE   SIGNIFICANCE   OF   THE   GERM    LAYERS. 


79 


certain  cases  the  fundamental  relations  of  the  three  layers, 
certain  portions  of  the  mesoderm,  for  instance,  failing  to 
differentiate  simultaneously  with  the  rest  of  the  layer  and 
appearing  therefore  to  be  a  portion  of  either  the  ectoderm 
or  endoderm.  But,  as  a  rule,  the  layers  are  structural 
units  of  a  higher  order  than  the  cells,  and  since  each  as- 
sumes definite  physiological  functions,  definite  structures 
have  their  origin  from  each. 

Thus  from  the  ectoderm  there  develop : 
/The  epidermis  and  its  appendages,  hairs,  nails,  epider- 
mal glands,  and  the  enamel  of  the  teeth. 

The  mucous  membrane  lining  the  mouth  and  the  nasal 
cavities,  as  well  as  that  lining  the  lower  part  of  the  rectum. 

The  nervous  system  and  the  nervous  elements  of  the 
sense-organs,  together  with  the  lens  of  the  eye. 

From  the  endoderm  develop: 

The  mucous  membrane  lining  the  digestive  tract  in 
general,  together  with  the  epithelium  of  the  various  glands 
associated  with  it,  such  as  the  liver  and  pancreas. 

The  lining  epithelium  of  the  larynx,  trachea,  and  lungs. 

The  epithelium  of  the  bladder  and  urethra. 

From  the  mesoderm  there  are  formed : 

The  various  connective  tissues,  including  bone  and  the 
teeth  (except  the  enamel). 

The  muscles,  both  striated  and  non-striated. 

The  circulatory  system,  including  the  blood  itself  and 
the  lymphatic  system. 

The  lining  membrane  of  the  serous  cavities  of  the  body. 

The  kidneys  and  ureters. 

The  internal  organs  of  reproduction. 

From  this  list  it  will  be  seen  that  the  products  of  the 
mesoderm  are  more  varied  l^f?n  those  of  either  of  the  other 
layers.  Among  its  products  are  organs  in  which  in  either 
the  embryonic  or  adult  condition  the  cells  are  arranged  in 


80 


THE    DEVELOPMENT    OF   THE    HUMAN    BODY. 


a  definite  layer,  while  in  other  structures  its  cells  are 
scattered  in  a  matrix  of  non-cellular  material,  as,  for  ex- 
ample, in  the  connective  tissues,  bone,  cartilage,  and  the 
blood  and  lymph.  It  has  been  proposed  to  distinguish 
these  two  forms  of  mesoderm  as  mesothelium  and  mesen- 
chyme respectively,  a  distinction  which  is  undoubtedly 
convenient,  though  probably  devoid  of  the  fundamental 
importance  which  has  been  attributed  to  it  by  some  em- 
bryologists. 

LITERATURE. 
R.  AssheTOn:  "A  Reinvestigation  into  the  Early  Stages  of  the  Develop- 
ment of  the  Rabbit,"  Quarterly  Journ.  of  Microsc.  Science,  xxxvii, 
1894. 
R.  Assheton:  "The  Development  of  the  Pig  During  the  First  Ten  Days," 

Quarterly  Journ.  of  Microsc.  Science,  XLI,  1898. 
R.   Assheton:  "The    Segmentation  of  the    Ovum   of  the    Sheep,  with 
Observations  on  the   Hypothesis  of  a   Hypoblastic  Origin  for  the 
Trophoblast,"  Quarterly  Journ.  of  Microsc.  Science,  XU,  1898. 
E.  VAX  BenEDEn-:  "Recherches  sur  Ics  premiers  stades  du  developpement 

du  Murin  (Vespertilio  niurinus),"  Anatom  Anzeigcr,  xvi,   1899. 
R.  Bonnet:  "Beitriige  zur  Embryologie  der  Wiederkauer  gewonnen  am 
Schafei,"    .\rchiv   fur   Amit.    und  Physiol.,   An^tt.    Abtli.,    1884   and 
1880. 
R.  Bonnet:  "Beitrage  zur  Embryologie  des  Hundes,"  Anat.  Heftc,  ix, 

1897 
Born:    "  Erste    Entwickelungsvorgiinge,"  Ergcbnissc   der    Anat.  und 

Entd'irkluvf^sgcsch.,  I,  1892. 

C.    EvclEshvmEk:   "The   Early   Development   of  Amblystoma   with 
Observations  on  Some  Other  Vertebrates,"  Journ.  of  Morf>hol.,  x, 

1895. 
H.xtschEk:   "Studi'.n  iiber   Entwioklung  des  Amphioxus,"   Arbeitcn 

aus  dem  znoloi^.  Instil,  zu  Wicn,  iv,   1881. 
W.  He.m'E:  "The  Development  of  the  Mole  (Talpa  europsa),"  Quarterly 

fourv.  of  .Microsc.  .SViVwd',  .XXIII,   1883. 
A.  A.  \V,  HiBKECiiT:  "Studies  on  .Mammalian  Embryology  II:  The  De- 

veloi)iiicnt   of   the   fk-riiiinal    Layers   of   Sorex   vulgaris,"    Quarterly 

Journ    of  Microti .  .Scunr, ,  xxxi,  1890. 
I'.   Keiheu:  "Studieii  /ur  Entvvick.  .jngsgesehichte  des  Schweines,"  Mor- 

pholoft,.  Arhritin,  111,  !8').V 
K.   MiTsi  KiTKi  and  C.   Ishik.vvv.a:  "On  the   Formation  of  the  Germinal 

Layers  in  Chelonia,"  Quarterly  Journ  of  .Microsc.  Science,  xxvii,  1887. 


G. 


A. 


B. 


iLi 


LITERATURE. 


8i 


E    SblEnka:  •  Studien  iiber  Entwickelungsgeschichte  der  Thiere."  4tes 

Heft,  188(,-87;5tes  Heft,  1891-92. 
I    Sobo«a:  ■•  Die  Befruchtung  und  Furchung  des  E.es  der  Maus.    Archzv 

jur  rrakrosk.  Aruil.,  XLV,  1895.  .      „    ^^      .     •        ^        ,      , 

J    SoBOTTA-.  "Die  Furchung  des  Wirhelthiereies,"  Ergebmsse  der  Anat. 

und  Entwickelungsgeschichte.,  vi,  1897.  ,.    ^     ,  „ 

L    Will:  "Beitrage  zur  Entwicklungsgeschicl.te  der  Reptilien,     /oolog. 
lahrhucher,  Ahth.  }ur  Anat.,  vi,   1893. 


■i 


I 


CHAPTER  III. 

THE  DEVELOPMENT  OF  THE  EXTERNAL 
FORM  OF  THE  HUMAN  EMBRYO 

The  youngest  human  ovum  at  present  known  is  that 
described  by  Peters.     It  was  taken  from  the  uterus  of  a 


ce~^ 


CJll. 


Fig.  ,^5.  -  Skctk)N  of  I^mbkvo  .wd  Aiijali;\t  Puktion  of  .\.n  Ovtm  of 

I     MM. 

i/m,  Amniotic  cavity;  n,  ciiorionic  cctDiicrm;  cm,  chorionic  mesoderm ; pf, 
embryonic  ectoderm;  i /;,  endoderm ;  m,  eiiihryonic  mesoderm;  ys, 
yolk-sac-  (I't'krs.) 


woman  who  liad  committed  suicide  one  calendar  month 
after  tlie  last  menstruation,  and  it  measured  about  i  mm. 
in  diameter.  The  entire  inner  surface  of  the  trophoblast 
(Fig.  35,  ce)  was  lined  by  a  layer  of  mesoderm  {an),  which, 

82 


THE  EXTERNAI.  FORM  OF  THE  HODV. 


83 


on  the  surfact  furthest  away  from  the  uterine  eavity,  was 
considerabiv  thicker  than  elsewhere,  forming  an  area  of 
attachment  of  the  embryo  to  the  wall  of  the  ovum.  In 
the  substance  of  this  thickening  was  the  amniotic  cavity 
(am),  whose  roof  was  formed  by  flattened  cells,  which,  at 
the  sides,  became  continuous  with  a  layer  of  columnar  cells 
forming  the  floor  of  the  cavity  and  constituting  the  em- 
bryonic ectoderm  (ec).     Immediately  below  this  was  a 


36. — Diagrams  to  show  the  PKt)BABLE  Relationsihps  of  the 
Parts  in  the  Kmbrvos  Represented  in  I'u.s.  28,  C,  and  33. 
Ac,  An:ni()tic  cavity;  C,  extra-embryonic  hiidy-cavily ;  A/i,  (in  ii^nrc  to 
the  k-fi)  tnesotlerm,  (in  figure  to  the  right)  somatic  mesoderm ;  .\/<', 
splanchnic  uicsodenii ;  /J,  digestive  tract;  En,  endodenii ;  7",  tropho- 
hla.'^i.  The  broken  Hne  in  the  mesoderm  of  the  figure  to  the  left  in- 
dicates the  line  along  which  the  splitting  of  the  mesoderm  occurs. 


layer  of  mesoderm  (m)  which  split  at  the  edge  of  the  em- 
bryonic disk  into  two  layers,  one  of  which  became  con- 
tinuous wnth  the  mesodermic  thickening  and  so  with  the 
layer  of  mesoderm  lining  i^he  interior  of  the  trophoblast, 
while  the  other  enclosed  a  sac  lined  by  a  layer  of  endo- 
dcrmal  cells  and  termed  the  yolk-sac  (ys).  The  total 
length  of  the  embryo  was  0.19  mm.,  and  so  far  as  its  ecto- 
derm and  mesoderm  are  concerned  it  might  l)e  described 
as  a  flat  disk  resting  on  the  surface  of  the  yolk-sac,  thougli 


I  ) 


iiil 


liii 

IP: 


84 


THE    DEVELOl'MENT    OF    THE    HUMAN    BODY. 


it  must  be  understood  that  the  yolk-sac  also  to  a  certain 
extent  forms  part  of  the  embryo. 

This  embryo  seems  to  be  in  an  early  stage  of  the  primi- 
tive streak  formation,  before  the  development  of  the  head 
process.     On  comparing  it  with  the  ovum  of  a  bat  in  ap- 
l)roximately  the  stage  of  development  represented  in  Fig. 
2S,  C,  it  will  be  seen  to  present  some  important  advances 
(Fig.  36).     It  seems  clear  that  the  yolk-sac  is  equivalent 
to  what  was  the  cavity  of  the  ovum  in  the  earlier  stages, 
and  consequently  the  cavity  (c)  into  which  the  yolk-sac 
projects  is  unrepresented  in  the  bats  ovum.     How  this 
cavity  is  formed  can  only  be  conjectured,  but  it  seems 
probable  that  it  arises  by  the  splitting  of  the  layer  of  cells 
which  lines  the  interior  of  the  trophoblast  in  the  bat's 
ovum  (or  perhaps  by  the  vacuolization  of  the  central  cells 
of  this  laver)  and  the  subsequent  accumulation  of  fluid 
between  the  two  mesodermal  layers  so  formed.     How- 
ever that  may  be,  it  seems  clear  that  the  size  of  the  human 
ovum  is  due  mainly  to  the  rapid  growth  of  this  cavity, 
which,  as   future  stages  show,  is   the  extra-embryonic 
portion  of  the  l>ody-cavity,  the  splitting  or  vacuolization 
of  th«-  mesorlerm  by  which  it  is  probably  formed  being  the 
precocious  appearance  of  the  typical  splitting  of  the  meso- 
derm to  form  the  embr\onic  body-cavity  which,  as  will 
be  seen  in  a  subsequent  chapter,  takes  place  only  at  a 
later  stage  of  development.     From  now  on  the  tropho- 
blast and  tilt:  lityi^r  of  mesoderm  lining  it  may  together  be 
spoken  of  as  the  chgriun,  the  mesoderm  layer  being  termed 
the  chorionic  mesoderm. 

.\  human  embryo  of  a  somewhat  greater  age  (Fig.  37). 
measuring  about  0.37  nmi.  in  length,  has  been  described 
bv  Graf  Spec  as  eml)ryo  r.IL,  and  was  taken  from  an 
o  urn  estimaied  to  measure  6  by  45  mm.  in  diameter. 
Notwithstanding   the   nmch   greater  size   of   the   ovum, 


■"^ 


THE  EXTERNAL  FORM  OF  THE  BODY. 


85 


which  is  due  to  the  continued  increase  in  the  size  of  the 
extra-embryonic  coelom,  the  embryo  is  but  little  advanced 
beyond  the  stage  which  the  Peters'  embryo  had  reached, 
and  is  probably  in  a  late  stage  of  the  development  of  the 
primitive  streak.  Confining  the  attention  for  the  present 
solely  to  the  embryo  and  the  immediately  adjoining  parts, 
it  will  be  seen  that  the  thickening  of  the  chorionic  meso- 
derm which  encloses  the  amniotic  cavity  has  increased  in 


Fi(..  ,A7.— Ovi  M  Measuring  6  :< 
4.5  MM.  The  Left  H.\lf  of 
THE  Chorion  has  Been  Re- 
moved TO  show  the  Kmbryo. 

a,  .\nini()tic  cavity:  /',  belly-stalk; 
c,  chorion ;  <-,  etnhryotiic  disk ;  v, 
chorionic  villus;  y,  yolk-sac. — 
{von  Sf>cc.) 


Fig.  38. -Kmbryo  1.54  mm.  in 
LENtiTH,      from     the     Dors.\l 

St'RF.\CE. 

a,  Amnion;  m,  medullary  groove; 
lie,  neurenteric  canal;  ps,  primi- 
tive streak;  v,  yolk-sac— (ron 
spec.) 


size  and  now  forms  a  pedicle,  known  as  the  belly-stalk  (b), 
at  the  extremity  of  which  is  the  yolk-sac  (;).  Further- 
more, the  amniotic  cavity  (a)  now  lies  somewhat  excen- 
trically  in  this  pedicle,  being  near  what  may  be  spoken  of 
as  its  anterior  surface.  The  embryo  still  possesses  a  dis- 
coidal  form  and  may  still  be  described  as  a  flat  disk  float- 
ing on  the  surface  of  the  yolk-sac. 

This  same  general  form  is  preserved  in  another  embryo, 
known  as  embryo  Gle,  described  by  Graf  Spec,  which 


da* 


86 


TllK    DKVELOPMKNT    OK   THE    HUMAN    W)1>Y. 


measured  1.54  mm.  in  length  (Fig.  38).  In  it,  however, 
the  more  median  portion  of  the  embryonic  disk  has  be- 
come thicker  and  is  separated  from  the  more  peripheral 
portions  by  a  distinct  furrow.  From  the  more  median  or 
axial  portion  the  embryo  proper  will  develop,  and  this  por- 
tion is  now  shaped  somewhat  like  the  body  of  a  violin  and 
presents  at  its  posterior  portion  the  remains  of  the  primi- 


I?,r,.    39.  —  DiAC.KAMS     IlUSTKATIN<;    TllK    CoNSTKlLTlON    1)1-    THE    E.MBRYO 

KKDM    THE    VoUK-SAC. 

.4  and  Care  limgitiulinal,  and  B  and  I)  transverse  sections.      1-1  is  drawn 
to  a  larger  scule  than  the  other  figures. 


!:> 


live  streak,  near  the  anterior  end  of  which  is  a  distinct 
pore,  the  opening  of  what  is  termed  the  neurcnterir  canal 
fuc),  a  description  of  which  will  be  found  in  a  subsequent 
chapter  ip.  i  i2j.  More  anteriorly  two  longitudinal  ridges 
have  appeared,  the  first  indications  of  which  are  termed 
the  medullary  folds 

In  later  stages  a  separation  or  constriction  of  the  em- 
brvo  from  the  volk-sac  begins  and  results  in  the  trans- 
formation of  the  discoidal  embryonic  portion  of  the  em- 


THi:    KXTERNAL    l-OKM    (»F    TIIK    ItODV. 


n; 


bryonic  disk  into  a  cylindrical  structure,     rriniarily  this 
depends  upon  the  deepening  of  the  furrow  which  surrounds 
the  embryonic  area,  the  edges  of  this  area  being  thus  bent 
in  on  all  sides  toward  the  yolk-sac.    This  bending  in  pro- 
ceeds most  rapidly  at  the  anterior  end  of  the  body,  as 
shown  in  the  diagrams  (Fig.  39),  ^mtl  least  rapidly  at  the 
posterior  end  where  the  belly-stalk  is  situated,  and  pro- 
duces a  constriction  of  the  yolk-sac,  the  portion  of  that 
structure  nearest  the  embryonic  disk  becoming  enclosed 
within  the  body  of  the  embryo 
to   form   the  digestive  tract, 
while   the  remainder  is  con- 
verted into  a  pedicle-like  por- 
tion, the  yolk-stalk,  at  the  ex- 
tremity of  which  is  the  yolk- 
vesicle.     The  further  continu- 
ance of  the  folding  in  of  the 
edges  of  the  embryonic  area 
leads  to  an  almost  complete 
closing    in    of    the    digestive 
tract  and  reduces  the  opening 
through  which  the  yolk-stalk  and  belly-stalk  communicate 
with  the  embryonic  tissues  to  a  small  area  known  as  the 
umbilicus. 

An  embryo  which  exhibits  an  early  stage  in  the  process 
of  constriction  has  been  described  by  Allen  Thompson  and 
is  represented  in  Fig.  40.*  It  measured  about  2.5  mm.  in 
length  and  had  readied  a  stage  in  which  the  medullary 
folds  had  become  very  pronounced  and  their  edges  had 
come  into  contact  at  one  po*  tion,  although  the  anterior 
and  posterior  portions  of  the  groove  (mg)  between  them 


Fia.  40-  Kmbryo  2.S  mm.  Long. 

<iw,  Fra>;nient  of  the  torn  am- 
nion; tng,  medullary  groove; 
Y,  V'tlk-sac. — (Aiiin  Thomp- 
son.) 


*  It  must  be  noted  that  in  the  figure  .u-iiliei  the  amnion  (exctpt  lor  u 
small  fragment  still  pcrsistmg  in  front)  nor  Uie  belly-stalk  is  rej^resentcd. 


tBCT-.jae-' 


if 


!i 


i| 


88 


TMK    DF.VF.roPMENT   OF   THE    HUMA»'    BOHY. 


were  still  widely  open.  The  embrvo  will  be  seen  from  the 
figure  to  project  somewhat  lioth  in  front  of  and  behind  the 
yolk-sac,  although  the  greater  part  of  its  ventral  surface  is 
still  formed  by  that  structure.  At  the  sides  also  it  is  well 
separated  from  the  yolk  sac,  and  resting  upon  the  sac  in 
front  is  a  swelling  which  represents  the  heart. 

In  another  embryo  (Fig.  41),  slightly  smaller  though 


fns 


am 


I'ld.  41. — Reconstruction  of  K.mbrvo  2.11  mm.  Lonc;. 

(i/,    .\llantnis;   am,  amnion;    B,   belly-stalk;    ch,  chorion;    h,   heart;    mx, 

inp,s(jdermic     somite;    os,    oral    fossa;     ph,  pharynx;     ~c,    chorionic 
villi;   Y,  yolk-sac. — (Ajlcr  Ettmod.) 

evidently  older  than  the  preceding  one,  and  described 
by  Eternod,  the  edges  of  the  medullary  folds  liave  not  only 
come  into  contact  throughout  the  greater  portion  of  their 
length,  but  they  have  fused  together,  the  groove  between 
them  being  open  only  in  front  and  behind.  On  each  side 
of  the  median  line  eight  somewhat  oblong  areas  are  to  be 


TIIF.    KXTEKNAI.    lOKM    OF    TIIK    IIOI>V. 


89 


distinguislicd,  caused  by  a  transverse  division  of  the  sub- 
jacent tncsoderni  into  what  ire  termed  mesodcrmic  somites 


m 


am-^ 


Vie  42.     Kmbryo  2.5  mm.  Lon'<;. 
am,   .Amnion;    B,  helly-stalk;  h,    heart;    M,  closed,  and    .)/',  still  ojien 
])nrlions  of  the  medullary   j;roove;  ('»«,   omplialomesenteric    vein; 
( '.S',  oral  foss;i;  1',  volk-Siic.      {KoUmanii) 


(ms),  structures  which  will  i)e  described  in  detail  in  the 
succeeding  chapter.     The  separation  of  the  embryo  from 


3*" 


1.0 


I.I 


1.25 


118 


1^ 

Hi 

Hi. 

1^ 

m. 

Ih 

2.5 

m 

2.0 
1.8 


MICROCOPY  RESOLUTION  TEST  CHART 

NATIONAL  BUREAU  OF  STANDARDS 

STANDARD  REFERENCE  MATERIAL  1010a 

(ANSI  and  ISO  TEST  CHART  No.  2) 


i  f 


90 


THE    UEVEI.OJ'.MKNT    OK    THE    HLM.W    liODV. 


!   ! 


H' 


i'   ! 
S    f 

I; 


}K-, 


tlie  j'oII-'-sac  (Y)  has  advanced  considerably  and  the  sac 
shows  evident  indications  of  constriction  just  where  it 
meets  the  body  of  the  embryo.  The  liead  projects  more 
markedly  beyond  the  anterior  surface  of  the  yolk-sac  and 
is  separated  from  the  region   occupied  by  the  heart  (h) 


'*}^^'^^ 


Vir,,  ■^^.     I'Imbkvo  L^',  2.15  MM.  I.ONf;. 
am,  Aiiinum;  li,  hdly-stalk ;   ('.chorion;   /;,  heart;    Y,  yolk-sac— (Wm.) 


by  a  deep  and  well-marked  depression,  the  oral  fossa  (os). 

In  an  embryo  described  by  Kollmann  (Fig.  42)  and 
measuring  2.5  mm.  in  length,*  the  edges  of  the  medullary 
folds  (M)  had  come  into  contact  throughout  their  entire 

*  The  embryo  was  measured  only  after  having  been  preserved  in 
alcohol,  and  the  actual  length  was  probably  somewhat  greater  than  this. 


THE    EXTERNAL    lORM    OF   THE    I!OI«\. 


91 


Icnijth,  except  for  a  short  distance  anteriorly  (A/M,  and 
thirteen  mesodennic  somites  were  visible.  The  constric- 
tion of  the  yolk-sac  was  even  more  proi  ounced  than  in 
the  preceding  embryo  and  the  hind  end  of  the  body  had 
become  defmcd,  the  belly-stalk  no  longer  seeming  to  be  a 
posterior  continuation  of  the  body  but  arising  irom  the 
posterior  part  of  the  ventral  surface.  The  oral  fossa  (OS) 
was  also  more  marked,  and  it  may  be  noticed  that  the 
dorsal  surface  of  the  body  was  distinctly  concave  from 
before  backward,  a  peculiarity  which  becomes  more  pro- 
nounced in  a  later  stage  and  constitutes  what  is  termed 
the  dorsal  flexure. 

This  is  well  shown  in  an  embryo  described  by  HiF  and 
named  by  him  embryo  lxviii  (Lg)  (Fig.  43)-     I"  i^  the 
yolk-sac  forms  a  much  smaller  portion  of  the  ventral  sur- 
face than  it  did  in  earlier  stages,  and  it  has  also  become 
distinctly    separated    from    the    belly-stalk.     The    most 
peculiar  feature  of  this  embryo  is,  however,  the  dorsal 
flexure.     This   is   apparently   a  normal   feature   and  is 
probably  produced  by  a  difl"erence  in  the  rate  of  growth 
of  the  lateral   and   median  portions  of  the  outer  layer 
of  the   embryonic   mesoderm,  the  former   portion    fail- 
ing to  keep  pace  with  the  growth  of   the  latter,  which 
becomes  folded  in  accommodation  to  the  strain.      The 
flexure  is   of  comparatively  short   duration,  and   when 
once  it  begins  to  disappear  it  seems  to  do  so  rapidly,  the 
dorsal  concavity  suddenly  becoming  a  convexity  and  the 
tension  of  the  layer  coming  into  equilibrium  in  the  new 
position.     One  other  feature  is  noteworthy  in  this  em- 
bryo—namely, the  occurrence  of  two  linear  vertical  dc 
pressions  a  little  behind  the  head  region  of  the  embryo; 
these  are  the  first  representatives  of  a  series  of  branchial 
clefts. 

These  structures  are  of  great  morphological  importance, 


92 


THE    DF.VELOI'MEXT    OF    THE    IllMAN    HODV. 


i 


!3  h 
Ml  i 


ll 


t  % 


■  f 


M 


inasmuch  as  they  determine  to  a  large  extent  the  arrange- 
ment of  various  organs  of  the  head  region.  They  repre- 
sent the  clefts  which  exist  in  the  walls  of  the  pliar  .-nx  in 
fishes,  through  which  water,  taken  in  at  the  mouth,  passes 
to  the  exterior,  bathing  on  its  way  the  gill  filaments 
attached  to  the  burs  or  arches,  as  thev  are  termed, 
which  separate  successive  clefts.  Hence  the  name 
"branchial"  which  is  applied  to  them,  though  in 
the    mammals  they    never   liave   respiratory   functions 

to  perform,  but,  appearing, 
persist  for  a  time  and  then 
either  disappear  or  are  ap- 
plied to  some  entirely  dif- 
ferent purpose.    Indeed,  in 
man  they  are  never  really 
clefts   but  merely  groove  \ 
and  corresponding  to  eac 
groove    in     the    ectoderm 
there  is  also  one  in  the  sub- 
jacent  endoderm    of   what 
will     eventually     be      the 
pharyngeal  region  of  t'e  di- 
gestive tract,  so  that  in  the 
region  of  each  cleft  the  ecto- 
derm and  endoderm  are  in  close  relation,  being  separated 
only  by  a  very  thin  layer  of  mesoderm,  while  in  the  inter- 
vals between  successive  clefts  a  m.ore  considp—ble  amount 
of  mesoderm  is  present  (Fig.  44). 

In  the  human  embr3-o  four  clefts  develop  in  each  side  of 
the  body  and  five  branchial  arches,  the  last  arch  lying 
posteriorly  to  the  fourth  cleft  and  not  being  very  sharply 
defined  along  its  posterior  margin. 

As  just  stated,  the  defls  are  normally  merely  grooves,  and 
m  later  development  either  disappear  or  are  converted  into 


Fig.  44. — Floor  of  thk  riiAKVNx 
OK  H;mbryo   H,   7  MM  LoNi;. 

Ep>  KpiRlDttis;  .S'/),  sinus  pra'cervi- 
calis;  t\  anterior,  and  f^,  pos- 
terior [Kirtions  of  the  tongue;  /, 
//,  ///,  and  /r,  hrancliial  arches' 


■  -a 


• » 


TIIK    KXTKKNAL    l-OKM    (>K    THK    IJODV. 


93 


Fig.  45. — Embryo  Lr,  4.2  mm.  I.g.so. 

ant,  Amnion;  au,  auditory  capsule;   B,  belly-stalk;   /(,    heart;   /./,    lower 

and  l.'l,  upper  limb;  V,  yolk-sac. — (His.) 


4      :f 


94 


THE    DKVM.Ol'MKNT    OK    TMK    IILMAN    liOOV. 


!f ;: 


.fl. 


li^lt! 


spfcial  striK-turis.  Occasionally,  however,  a  cleft  niav  jHTsist 
and  the  thni  membrane  which  forms  its  lloor  may'  become 
perforated  so  that  an  opening  from  the  exterior'  into  the 
pharynx  occurs  at  the  side  of  ilie  neck,  forming  what  is  termed 
a  branchuil  fistula.  Such  ati  abnormality  is  most  frecpientlv 
developetl  from  the  lower  (ventral)  part  of  the  first  cleft; 
normally  this  disapjiears.  the  upper  portion  persisting,  how- 
ever, to  form  the  external  auditorv  meatus  and  tvmnanic 
cavity.  "      ' 

The  etTibryo  lxviii  (Lj?)  just  descril)C(l  measured  2.11 
mm.  in  length,  this  measurement,  however,  being  taken 
along  a  straight  line  and  not  following  the  flexure  of  the 
body.     It  does  not  represent,  therefore,  the  actual  length 
of  the  body  and  there  is  much  less  difference  between  it 
and  the  next  embryo  described  than  is  implied  by  the 
figures.     This  embryo  (Fig.  45)  is  also  one  of  those  de- 
scribed by  His  and  is  known  as  embryo  i^xvii  (Lr).     It 
measures  4.2  mm.  in  length  and  shows  an  almost  complete 
disappearance  of  the  dorsal  flexure  so  marked  in  embryo 
Lxviii.     Instead  of  this,  it  presents  a  well-marked  ventral 
bending  of  both  the  anterior  and  posterior  portions  of  the 
body,  so  that  the  dorsal  surface  is  prominently  convex  in 
the  regions  which  will  later  be  the  nape  of  the  neck  and  the 
sacral  region,  and  consequently  the  convexities  may  be 
known  as  the  neck  bend  and  the  sacral  bend.    Furthermore, 
theie  is  noticeable  a  ventral  projection  of  the  extreme 
front  end  of  the  body,  so  that  a  third  convexity  occurs 
anteriorly  to  the  neck  bend  and   may  be  termed  the  head 
bend. 

The  constriction  of  the  yolk  sac  has  progressed,  as 
has  also  its  separation  from  the  belly-stalk;  the  meso- 
dermic  somites  have  almost  reached  their  maximum 
development  and  are  very  distinct;  the  two  branchial 
clefts  present  in  the  preceding  embryo  have  increased  in 
size  and  the  third  cleft  has  made  its  appearance;  two 


i^i 


IIIK    KXTKKNAI.    I(»'<M    OF     IIIK    I'.ODV. 


95 


Fio.  46      Embryo  of  from  Twenty  to  Twenty-fivE  Days. 

.4  w,  Amnion;  LL,  lower  limb ;  t/.4,  uml)ilical  artery;    fc,  umbilical  cord; 

L'L,  upper  limb;  Vj-,  yolk-sac. — {Costc.) 


;    s  ,i! 


^  ■! 


96 


Tlir.    DKVEl.OPMKNr    (il      IIIK    lUM AN    ItoDV. 


small  t'levations  of  the  sides  of  the  body,  one  almost  oppo- 
site the  neck  bend  and  the  other  opposite  the  sacral  bend, 
are  the  first  indications  of  the  limbs  ( (7  and  LI) ;  and  the 
eyeball  and  ear  vesicle  Uiu),  which  were  present  though 
not  very  evident  in  earlier  stages,  are  now  plainly  visible 
in  surface  views. 

In  the  next  stage-as  a  type  of  which  an    embryo 


II 


ili 


^  ;,  i 
1  i    ••  i 

Hi 

u/^ 


Fig.  47. — Kmbryo  9.1  mm.  I.o.ng. 
LI,  I.cwer  limb;  I',  iinihilical  cord;  I'l,  upper  litiih;  V,  yolk-sac. — (His.) 

figured  by  Coste  (Fig.  46)  may  be  taken — the  three  bends 
of  the  body  mentioned  above  have  greatly  increased,  so 
that  the  head  and  tail  of  the  embryo  are  almost  in  contact 
and  the  latter  is  bent  a  little  toward  one  side.  The  closure 
of  the  ventral  surface  of  the  body  is  almost  completed  and 


mi 


TMK    KMKKNM.    HOKM    OK    THK    .H»1'V. 


97 


"u,ora„W  in  length  an,.  ^^^  ^:X^y^^^ 
eiubrvonic  portions  into  a  yolk  stalk  ana  >o  k 
i^    V    dist^nguishabk      The  limb  ruduncnts  have  n - 
c  ea  c^  sotncvvhat  in  size.  and.  in  addition  to  the  eyeball 
and  ear  ^"ele.  a  third  sense-organ  has  made  Us  appear^ 
aneeTn  the  orm  of  two  nits  situated  on  the  under  side  o 
the  anterior  portion  of  the  head;  these  pits  a      die  first 
indications  of  the  «a^u/M^«. 

The  fourth  branchial  cleft  has  appeared  and  those 
formed  earlier  have  elongated  so  that  they  al-J^-- 
t'  .  mid-ventral  line,  and  from  the  dorsal  part  of  the  ante 
norbler  of  the  first  arch  a  strong  process  has  developed 
so  that  the  arch  on  each  side  is  somewhat  <-shap  d-      11^ 
upper  limb  of  each  V  is  destined  to  give  rise  to  the  upper 
•aw   and  hence  is  known  as  the  ---^^-^/-;;;^  J^^^^^^^^ 
the  lower  limb  represents  the  lower  jaw  and  is  termed  the 

mandibular  process.  ^„*:„„  ^f  the 

Leaving  aside  for  the  present  all  consideration  of  the 

further  development  of  the  limbs  and  branchial  arches. 

the  urther  evoLion  of  the  general  form  of  the  body  may 
rapidly  sketched.     In  an  embryo  (Fig.  47)  j-m  Ruge  s 

collection,  described  and  figured  by  His  and  measuring 
9. 1  mm.  in  length,*  the  prolongation  of  the  margins  of  the 
umbilicus  has  increased  until  more  than  half  the  yolk- 
stalk  has  become  enclosed  within  the  umbilical  cord^  1  he 
neck  and  sacral  bends  are  still  very  pronounced,  although 

"   Trhis^rement  is  taken  in  a  straight  line  from  t^enu.st  anterior 
portion  of  the  neck  bend  t.  the  middle  point  of  the  sacral  b-d  and  d 
not  follow  the  curvature  of  the  embryo.     It  may  be  spoken  of  as  the  neck 
rmnp  length  and  is  convenient  for  use  during  the  stages  when  the  embryo 
is  coiled  upon  itself. 


98 


TilK    DF.VKI.OI'MKNT    OF'    TIIK    HUMAN    llODV. 


V  I 

■    '1' 
•I 


II  S' 


the  embryo  is  bej^i'iiiiiij,'  to  straijjjhU  ii  out  and  is  not  quite 
so  much  coiled  as  in  the  preceding  stage.  At  the  poste- 
rior end  of  the  bodv  there  has  developed  a  rather  abruptly 
conical  tail  filament,  in  the  place  of  the  blunt  and  gradu- 
ally tapering  termination  seen  in  earlier  stages,  and  a  well- 
marked  rotundity  of  the  abdomen,  due  to  tl  e  rapidly  in- 
creasing size  of  the  liver,  begins  to  become  evident. 

In  later  stages  the  enclosure  of   the  yolk-  and  belly- 
stalks  within  the  umbilical  cord  proceeds  until  linallv  the 


Fro.  48.    -Embryo  Hfj,  \^.f,  mm.  I.onc. —(His.) 


ii 

It ' 
ill. 


r 


cord  is  complete  through  the  entire  interval  between  the 
embryo  and  the  wall  of  the  ovum.  At  the  same  time  the 
straightening  out  of  the  embryo  continues,  as  may  be  seen 
in  Fig.  48  representing  the  embryo  xi.v  (Br^)  of  His,  which 
shows  also,  both  in  front  of  and  behind  the  neck  bend,  a 
distinct  deprc^^ion,  the  more  anterior  one  being  the  occi- 
pital and  the  u.jre  posterior  the  neck  depression;  both 
these  depressions  are  the  expressions  of  changes  taking 


TlIK    KMKKNAl     KOKM    OF    TIIK    HOI'V. 


99 


place  in  the c.tral  ,u«rvous  systcu.     TIk-  ta.l  Llanu.,   has 
W„      .ncrc  ,nark.l.  ami  \u  tlu-  lua<l  rc,u.,.  a  sh,   1 
;•    "  urroumlin,  tlu-  cy.l.all  and  n.arkinK  ont  the  con- 
;!:;tival  area  has  appeared,  a  depress.n.  ^-^"or  U.  Ok 
uisal  fossa-  nuirks  olT  the  nose  from  ^^'^  /  ''^^''^; '   .;;  ' 
tL  external  eat.  whose  developn.ent  will  he  co.isuler  d 
later  on.  has  heconK-  <pute  distinct.     This  en.bryo  had  a 
neck-rump  length  of  !  3.6  mm. 


f  ,.    4.;    ^      I.  KmBKVC.  S,.  15  MM.  I-ONC;  (snoW.N.;  HCTO.MA  nV  TUK  HKART)  ; 
n,  Kmbryo  1,3:.   17. .S  MM.   LONC.      {H'S.) 

In  the  embryos  XXXV  fS.,)  and  xcix  (L,)(T?ig  49,  A  and 
B)  of  His'  collection  .he  straightening  out  of  the  neck  bend 
is  proceeding,  and  indeed  is  almost  completed  in  embryo 
XCIX,  which  begins  to  resemble  closely  the  fully  formed 
fetus'.  The  tail  filament,  somewhat  reduced  in  size,  still 
persists  and  the  rotundity  of  the  abdon.eii  continues  to  be 
well  marked.     The  -   .k  region  is  beginning  to  be  distin- 


lOO 


TIIK    l)i:\  KI.OI'MI  N  r    (p|      I  UK    IHMAN     HmDV. 


K'uisliable  in  trnhryo  xxxv  and  in  tinhryo  xcix  tin-  t-ydids 
liavi'  apptartd  as  slight  folds  surrounding,'  tlu-  conjuncti- 
val area.  The  nose  and  forehead  are  clearly  defined  by 
th  reater  developnunt  of  the  nasal  j;roove  and  the  nose 
has  also  become  raised  al)ove  the  j^eneral  surface  of  the 


I 


M 


II' 


il 


ii^i;l 


Fig.  50.     Kmbryo  Wt,  2.^  .mm.  J.osv.'     (His.) 

face,  while  the  external  ear  lias  almost  acquired  its  final 
fetal  form.  These  embryos  measure  respectively  about 
15  and  17  -  -im.  in  length.* 

*  The  embryo  xxxv  pnsents  a  slight  abnormaHty  in  the  great  pro- 
jection of  the  heart,  but  otherwise  it  appears  to  be  normal. 


niK    KMKKNAI.    H)KM 


TlIK    IIOHV 


l"> 


,,i„,,K    a„  ..,„l,rvo    uKuin  one  of  tlu«  ,I«t,I,«1    - 
Mis  n  .n.lv.  .  -  ..xxv  (\Vt)  lu.vin>;  al™«'h  of  .,, ."...-_ 
,  ;.  ■     fe.r-1  I >••'«■  M.)  as  rcproscUiMK'  .1,.-  prac  .«,1 
:  „i  Uu      of  .h.  fCal  form.     This  ombryo  dales  fro„, 
:    ,m  the  en.l  of  Ibe  seeon.l  .nonth  of  preRnaney    a„.l 
,1      perio,l  omvurd  it  is  proper  to  use  the  term  e.us 
ra     e    than  .hat  of  en.bryo.     The  ehan«es  wh.ch  have 
en  lescrihcl  in  preceding  stages  are  nou  eonjplete  a    I 
r,nains  only  to  he  ntentione.l  that  the  eanrtal  hh.nu  „t 
lie     is  still  prominent,  Kra.lually  disappears  m  later 
;   ,  hecon,i!,«,  as  it  .  -e,  sul.n.c  -«■  a„,l  eoneeal«l 
,,„L;h  adjacent  parts  l.v  the  devc     ,ment  of  U.e  1  ut- 
tocks.     The  ineon.pleteness  of  the  developtnent  of  th  se 
regions  in  en.br.o  t.xxv..  is  ■•■  milest,  not  only  front  the 
nnieetion  of  the  ta  ■    ilanten,.  i.ut  also  front  the  ex- 
feri.  genitalia  heing  still  largely  visible  n>  a  sule  vtew 
of  the  embryo,  a  eondilion  whieh  will  d.-uppear  in  later 

stages. 

The  Later  Development  of  the  Branchial  Arches,  and  the 
Development  of  the  Face.     In  Costes  embryo  aMg.  46) 
the  four  branchial  clefts  and  five  arches  which  develop  in 
the  human  embryo  are  visible  in  surface  views  but  in  the 
Ruge  embrvo  (Kig.  47)  it  will  be  noticed  that  only  the  first 
two  arches;  the  first  with  a  v. ell-developed  maxillary  pro- 
cess and  the  cleft  separating  them  can  be  distinguished. 
This  is  due  to  a  sinking  inward  of  the  region  occupied  by 
the  three  posterior  arches  so  that  a  triangular  depression 
the  sinus  prcBCcrvicalis,  i.  lormed  on  eaJ.i  side  of  what  will 
later  become  the  anterior  part  of  the  neck  region.     This  is 
well  shown  in  an  embryo  (Pr,)  described  by  His  which 
measured  6.9  mm.  in  length  and  of  which  the  anterior  por- 
tion is  shown  in  Fig.  51-     Tl^^  interior  boundary  of  the 
sinus  ips)  is  formed  by  the  posterior  edge  of  the  second 
arch  and  its  posterior  boundary  by  the  thoracic  wall,  and 


I02 


THE    DEVELOPMENT    OF    THE    HUMAN    BODY. 


■   ii 

I 


in  later  stages  these  two  boundaries  gradually  approach 
one  another  so  as  first  of  all  to  diminish  the  opening  into 
the  sinus  and  later  to  completely  obliterate  it  by  fusing 
together,  the  sinus  thus  becoming  converted  into  a 
completely  closed  cavity  whose  floor  is  formed  by 
the  ectoderm  covering  the  three  posterior  arches  and 
the  clefts  separating  these.     This  cavity  eventually  im- 


I 
ill 


S^l  M 


If  I 


li^  ■'  i 


m 


Fk;.  51.— Head  of  Embryo  of  6.9  mm. 
na,  \asal  pit;  ps,  precervical  sinus. — {His.) 

dergoes  degeneration,  no  traces  of  it  occurring  normally 
in  the  adult,  although  certain  cysts  occasionally  observed 
in  the  sides  of  the  neck  may  represent  persisting  portions 
of  it. 

A  somewhat  similar  process  results  in  the  closure  of  the 
ventral  portion  of  the  first  cleft,*  a  fold  growing  back- 


*  See  page  94,  small  type. 


It^ 


THE    DEVELOPMENT    OK    THE    FACE. 


103 


ward  frotn  the  posterior  edge  of  the  first  arch  and  fusing 
Tviu'the  ventral  part  of  the  anterior  border  of  the  seeond 

''The  upper  part  of  the  second  cleft,  however,  persists, 
and  as  aSy  stated.forms  the  external  auditory  nr.eatus 
hep^na  of  the  ear  being  developed  from  the  ad3acent 
narts  of  the  first  and  second  arches  (Figs.  48  and  49).        , 
'  ?L  re^on^^^  in  f-nt  of  the  first  arch  .s 


///;— 


mxp 


mxp,  Maxillary    process 


iMG.  52.  -Face  of  Umbkyo  of  8  mm. 

tip,  nasal    pit;    os,   oral  fossa; 
globularis.    -(//i.v.) 


/)g,   processus 


occupied  by  a  rather  deep  depression,  the  oral  fossa,  whose 
eariy  development  has  already  been  traced.  In  an  em- 
bryo measuring  8  mm.  in  length  (Fig.  52)  the  fossa  (os) 
has  assumed  a  somewhat  irregular  quadrilateral  form. 
Its  posterior  boundary  is  formed  by  the  mandibular  pro- 
cesses of  the  first  arch,  while  laterally  it  is  bounded  by  the 


I04 


THE    DEVELOPMENT    OF    THE    HUMAN    UOOV. 


maxillary  processes  (mxp)  and  anteriorly  by  the  free  edge 
of  a  median  plate,  termed  the  nasal  process,  which  on 
either  side  of  the  median  line  is  elevated  to  form  a  marked 
protuberance,  the  processus  globularis  (pg).  The  ventral 
ends  of  the  maxillary  processes  are  viridely  separated,  the 
nasal  process  and  the  processus  globulares  intervening 


»' 


I 


I'lG     53  --l'\CE  OK   EmBKVO  AKTER  THE  COMPLETION  OF  THE  UpPER  JaW, 

■  '    ■  -(His.) 


ylf 


|)i  ! 


between  them,  and  they  are  also  separated  from  the  globu- 
lar processes  by  a  deep  and  rather  wide  groove  which 
anteriorly  opens  into  a  circular  depression,  the  nasal  pit 

(np). 

Later  on  the  maxillary  and  globular  processes  unite. 


Tin-:    KKVELUPMKS  !■   OF   THK    FACE. 


105 


oHitcratins  the  groove  and  cutting  oH  the  nasal  p.ts- 
wWch  have  by  thl  time  deepened  to  form  the  nasal  fossa, 
romdirect  communication  with  the  mouth,  wUh  whrch, 
to»rer  thev  still  communicate  behind  the  maxdlary 
iXs";,  an  indication  of  the  anterior  and  posterior 
nares  being  thus  produced. 

j--rort,.H^r'^cii;fa'-sr;,^t,^; 

known  as  "harelip." 

M  the  time  when  this  fusion  occurs  the  nasal  foss^  are 
widely  separated  by  the  broad  nasal  process  (Hg.  53).  bu 
du  ing  later  development  this  process  narrows  to  form 
t^nasal  septum  and  is  gradually  elevated  above  the 
.eneral  surface  of  the  face  as  shown  m  Figs  48-50.     By 
?he  narrowing  of  the  nasal  process  the  globular  processes 
a  e  brought  nearer  together  and  form  the  portions  of  the 
upper  jaw  immediately  on  each  side  of  the  median  hne, 
the  rest  of  the  jaw  being  formed  by  the  maxillary  pro- 
cesses     In  the  mean  time  a  furrow  has  appeared  upon  the 
mandibular  process,  running  parallel  with  its  borders  (Fig. 
40)  ■  the  portion  of  the  process  in  front  of  this  furrow  gives 
rise  to  the  lower  lip  and  is  known  as  the  lip  ndge,  while 
the  portion  behind  the  furrow  becomes  the  lower  jaw 
nroper  and  is  termed  the  chin  ridge. 

The  Development  of  the  Limbs.-As  has  been  already 
pointed  out,  the  Umbs  make  their  appearance  in  an  em- 
bryo measuring  about  4  mm.  in  length  (Fig.  45)  and  are  at 
fir'st  bud-Hke  in  form.  As  they  increase  in  length  they  at 
first  have  their  long  axes  directed  parallel  to  the  longi- 
tudinal axis  of  the  body  and  become  somevhat  flattened 
at  their  free  ends,  remaining  cylindrical  in  their  proximal 
portions.  A  furrow  or  constriction  appears  at  the  junc- 
tion of  the  flattened  and  cylindrical  portions  (Fig.  47).  and 
later  a  second  constriction  divides  the  cylindrical  portion 


I 


1 06 


THE    DEVELOPMKNT   OK    THE    HUMAN    liODV. 


i;ii .! 


into  a  proximal  and  distal  moiety,  the  three  segments  of 
each  lim',  -the  arm,  forearm,  and  hand  in  the  upper  limb, 
and  the  thigh,  leg,  and  foot  in  the  lower— being  thus 
marked  out.  The  digits  are  first  indicated  by  the  devel- 
opment of  four  radiating  shallow  grooves  upon  the  hand 
and  foot  regions,  and  a  transverse  furrow  uniting  the  prox- 
imal ends  of  the  digital  furrows  indicates  the  junction  of 
the  digital  and  palmar  regions  of  the  hand  or  of  the  toes  and 
body  of  the  foot.  After  this  stage  is  reached  the  develop- 
ment of  the  upper  limb  proceeds  more  rapidly  than  that  of 
the  lower,  although  the  processes  are  essentially  the  same 
in  both  limbs.  The  digits  begin  to  project  slightly,  but 
are  at  first  to  a  very  considerable  extent  united  together 
by  a  web,  whose  further  growth,  however,  does  not  keep 
pace  with  that  of  the  digits,  which  thus  come  to  project 
more  and  more  in  later  stages.  Even  in  comparatively 
early  stages  the  thumb,  and  to  a  somewhat  slighter  extent 
the  great  toe,  is  widely  separated  from  the  second  digit 
(Figs.  49  and  50). 

While  these  changes  have  been  taking  place  the  entire 
limbs  have  altered  their  position  with  reference  to  the  axis 
of  the  body,  being  in  stages  later  than  that  shown  in  Fig. 
47  directed  ventrally  so  that  their  longitudinal  axes  are  at 
right  angles  to  that  of  the  body.  From  the  figures  of  later 
stages  it  may  be  seen  that  it  is  the  thumb  (radial)  side  of 
the  arm  and  the  great  toe  (tibial)  side  of  the  leg  which  are 
directed  forward;  the  plantar  and  palmar  surfaces  of  the 
feet  and  hands  are  turned  toward  the  body  and  the  elbow 
is  directed  outward  and  slightly  backward,  while  the  knee 
looks  outward  and  slightly  forward.  It  seems  proper  to 
conclude  that  the  radial  side  of  ^  le  arm  is  horn-'  .gous 
with  the  tibial  side  of  the  leg,  the  palmar  surface  of  the 
hand  with  the  plantar  surface  of  the  foot,  and  the  elbow 
with  the  knee. 


<i    ! 


»|: 


•m 


THE 


DEVELOPMENT    OK    TMK    I.IM1«. 


107 


Tu.  limho  arc  however,  still  in  tlie  quadrupedal  condi- 

The  limbs  arc,  "°"        '      ,         ^  second  alteration  in 

tion,  and  they  mus   later  untog  ^.^,^ 

rS~e  L       TM:i^aec.„i,i:shed  by  a  rotation  of 

hip.  oints  toget  er  with  a  rota  ^^^^_ 

nal  a^s  through  -  -f  ,  "'^t  ^   L"^  '  "^  "P""^'"" 

of  the  leg  is  the  inner  side,  and  -vhereas  the  palmar  surlace 
j  the  tand  looks  ventrallv,  the  plantar  surface  of  the  foot 

'°t  S  these  statements  no  account  is  taken  of  the 
seconTary  position  which  the  hand  may  assume  as  the 
resuU  of  Us  pronation;  the  positions  given  are    hose  a  - 
sunTcd  by  the  limbs  when  both  the  bones  of  their  m.dd. 
segment  are  parallel  to  one  another. 

mmmmEm 

dorsally— is  termed  its  extension. 

T'5e  Age  of  the  Embryo  at  Different  Stages.-The  age  of 
an  .mbryo  mu.tbe  dated  from  the  tnonient  of  f c-rtilization 
and  from  what  ha',  been  said  in  pre vious  pages  (pp.  50,  5  ; 


i 


B 


io8 


THE    DKVELOI'MENT    OF    THE    II 


Ijn  1 


it  is  evident  that  it  must  be  exceedingly  difticu 
mine  the  exact  age  of  any  embryo  even  when  tht  ' 
the  cessation  of  the  menses  and  the  date  ui  the  cohabita- 
tion which  resulted  in  the  pregnancy  are  known.  And, 
furthermore,  not  only  is  the  actual  date  of  the  beginriug 
of  development  uncertain.but  in  the  majority  of  the  known 
human  embryos  in  early  stages  the  time  of  the  cessa- 
tion of  development  is  also  more  or  less  uncertain,  since 
the  embryos  are  abortions  and  their  expulsion  need  not 
necessarily  have  immediately  succeeded  their  death. 

These  various  sources  of  uncertainty  are  of  especial  im- 
portance in  the  early  stages  of  development,  when  a  day 
more  or  less  means  nmch.  But  nevertheless  it  is  conve- 
nient to  have  some  estimate  of  the  age  of  such  embryos 
even  though  it  be  recognized  that  iny  date  given  is  a  mere 
approximation.  H's  has  made  an  estimate  of  the  age  of  a 
number  of  embryos  concerning  which  approximate  data 
were  available  with  results  which  are  stated  in  the  fol- 
lowing table : 

At  2-2J  weeks  the  cMiil)ry()  measures  2.2  -  3      mm.  in  length. 

"    2J-3       "  "  "  -^    -  -i-S  mm. 

"      3i         "  "  "  .S    -  6      mm. 

"       4  "  "  "  7-8      mm.  " 

"       4i  '■  "  "  10    ~1!       mm. 

"       .s  "  "  "  13  mm.  " 

It  must  be  borne  in  mind,  however,  that  embryos  of  the 
same  age  need  not  in  all  cases  be  of  the  same  length,  since 
conditions  of  nutrition,  etc.,  will  largely  determine  not 
only  the  size  of  the  embryo,  but  also  the  amount  of  its 
devel  -nent.  And,  furthermore,  it  seems  probable  that 
the  Chimiates  for  age  given  in  the  above  table  may  be  too 
small,  since  there  is  reason  to  believe  that  the  earlier  stages 
of  development  proceed  more  slowly  than  do  the  later 
ones.     Thus,  BischofT  found  that  the  embryonic  disk  in 


3 
s 


••"K    GKOWTll    OK    THE    'MIIRYO.  IO9 

M  .iowcd  but  little  differentiation  ap  to  the  scv- 

•iiii  o.  eighth  day,  while  at  the  tenth  day  the  embryo 
possessed  branchial  clefts  and  mesodcrmic  somites.  It 
would  seem  from  the  available  data,  which  are  more 
definite  than  usual,  that  ?  human  embryo  described  by 
Hternod  and  measuring  only  1.3  mm.  in  length  was  very 
licarly  twenty-one  days  old ;  and  if  this  estimate  be  correct 
then  the  ages  assigned  by  His  to  the  e  rlier  embryos  must 
be  very  considerably  increased. 

As  regards  the  later  periods  of  development,  the  limits 
of  error  for  any  date  become  of  less  importance.  His 
estimates  that  at  the  end  of  the  second  month  when  the 
embryo  becomes  a  fetus,  its  length  is  about  J5  to  28  mm., 
and  for  later  periods  Schroder  gives  the  following  mea  ;ure- 
ments  as  the  average:  ^ 

^^l  lunar  niontli,    70-  90  ni..i. 

4th  '•  '•       100-170  mm. 

■■^th  "  "       180  270  mm. 

f^-t'i  "  "       280-340  mm. 

"til  "  "       350-380  mm. 

«tli  "  "       425  mm. 

ytl>  "  "  !..  467  mm. 

10th  "  "       490  500  mm. 

The  data  concerning  the  weight  of  embryos  of  different 
ages  are  as  yet  very  insufficient,  and  it  is  well  known  that 
the  weights  of  new-born  children  may  vary  greatly,  the 
authenticated  extremes  being,  according  to  Vierordt,  717 
grams  and  6123  grams.  It  is  probable  that  considerable 
variations  in  weight  occur  also  during  fetal  life.  So  far 
as  embryos  of  the  first  two  months  are  concerned,  the 
data  are  too  imperfect  for  tabulation;  for  later  periods 
Fehling  gives  the  following  as  average  weights  : 

3d  month,     20  grams. 

4tli        "        120        '• 

5th        "         285 


'i 


no  TIIK    DEVEI.OrMENT    OK    TIIK    HUMAN    BODY. 

6th  month, 6^5  grams. 

7th        "        1220       " 

8th        "        1700        •' 

'nh        •'        2240        " 

10th        "         3250       " 


1 

ii 


'i  • 


ii 


III 


'I  * 


LITERATURE. 
J.    Broman:  "  Beohachtung  eines  menschlichen   Embryos  von  beinahe  3 

mm.  I<ange  mit  specieller  Bemerkung  uber  die  bei  demselben  befmd- 

lichenHirnfulten,"  Morpholog.  Arbeiten,  v,  1895. 
J.   M.  Coste:  "Histoire  generale  et  particulifere  du  developpement  des 

corps  organises,"  Paris.  1847-1859. 
A.  KckEr:  "  Beitrage  zut  Kenntniss  der  iiusserer  Formen  jungster  mensch- 

licher   Embryonen,"    Archiv   }ur   Anal,   und  Physiol.,    Ariat.    Abth., 

1880. 
A.  C.  F.  Eternod:  "Communication  sur  un  <i'uf  humain  avec  embryon 

excessivement  jeune,"  Archives  Ital.  de  Hiologie,  .xxii,   1895. 
A.  C.  F.  Eternod:  "II  y  a  un  canal  notochordal  dans  I'embryon  humain," 

Atuit.  Ameiger,  xvi,   1899. 
C.  GiACOMiNi:  "Un  ceuf  humain  de  11  jours,"  Archives  Ital.  de   Biologie, 

XXIX,  1898. 
V.  HensEn:  "Beitrag  zur  Morphologic  der  Korijcrform  und  des  Gehirns 

des  menschlichen  Embryos,"  Archiv  jiir  Anat.  und  Physiol.,  Anat. 

Abth.,  1877. 
W.  His:  "Anatomic  nienschlicher  Embryonen,"  Leipzig,  1880. 
J.  Janosik:  "Zwei  junge  menscliHche   Embryimen,"  Archiv  jiir  mikrosk. 

Anat.,  XXX,  1887. 
F.   KeibEl:  "Ein  sehr  junges  menschlicher  Ei,"  Archiv  jiir    Atuit.  und 

Physiol.,  Aiuit.  Abth.,  1890. 
F.   KeibEU:  "Ueber  einen  menschlichen  Embryo  von  6.8  mm.  grosster 

Liinge,"  Verhandl.  Anatom.  Gescllsch.,  Xitl,  1899. 
I.  KoutMANN :  "  Die  KOrperform  menschlicher  normaler  und  pathologischer 

Embryonen,"   Archiv  jar  Anat.   und  Physiol.,  Anat.   Ab     ,  Supple- 

mcnt,  1889. 
F.  P.  Malu:  "A  Human  Embryo  Twenty-six  Days  Old,"  Journ.  oj  Mor- 
phology, V,  1891. 
F.  P.  Mall:  "A  Human  Embryo  of  the  Second  Week,"  Anat.  Ameigcr, 

VIII,  1893. 

F.  P.  Mall:  "Early  Human  Embryos  and  the  Mode  of  their  Preserva- 
tion," Bulhlin  oj  the  Johns  Hopkins  Hospital,  IV,  1894. 

C.  S.  MiNOT:  "Human  Embryology,"  New  York,  1892. 

J.  MOller:  " Zergliederungen  menschlicher  Embryonen  aus  fruherer 
Zcit.,"  Archiv  jUr  Anat.  und  Physiol.,  1830. 


LITERATUKE. 


Ill 


M! 


H.  Peters:  "Uelwrdie  Einbettung  des  inenschlichen  P^ies  und  das  friihcste 

bisher  bekannte  mcnschliche  Placentarstadiuin,"  Leipzig  und  Wien, 

1899. 
C.  I'MiSALix:  "  Ktude  d'un  Embryon  hutnain  de  II  niillitnct'-cs,  "  Archives 

de  ZKolog.  expcrimtniale  ct  gitiinik,  Sor.  2,  vi,  1888. 
H.  Piper:  "Ein  menschlicher  Embryo  von  6.8  mm.  Nackcnlinie,"  Arcliiv 

jar  Anat.  und  Physiol.,  Atiat.  Ahlh.,   1898. 
I".  Or.\k  vo.\  SfeE:  "  Beobachtunjjen  an  eincr  menschlichen  Keimscheilie 

niit  ofTener  MedulUirrinne  und  Canalis  neurentericus,"   Archiv  jiir 

Anat.  und  Physiol.,  Anat.  Abth.,   1889. 
V.  Gr.\f  von  Spee:  "Ueber  friihe  Entwickelungsstufen  des  menschlichen 

Hies,"  Archiv  jiir  Anat.  und  Physiol.,  Anat.  Abth.,  1896. 
Ai.uEN  Thompso.v:  "Contributions  to  the  History  of  the  Structure  of  the 

Human  Ovum  and  Embryo  before  the  Third  Week  after  Conception, 

with  a  Description  of  Some  Early  Ova,"  Edinburgh  Mid.  and  Surg. 

Journal,  in,  1839      (See  also  Eroriep's  \iuc  Notizen,  xiii,   1840.) 


I 


>       f 


I    i 


!l    i 


!      ! 
»   i      ! 


CHAI'TKR  IV. 

THE   MEDULLARY  GROOVE,  NOTOCHORD,  AND 
MESODERMIC  SOMITES. 

In  the  youngest  human  embryos  known,  such  as  the 
Peters'  embryo  and  the  youngest  embryo  described  by 
Graf  Spec,  there  is  no  differentiation  of  the  embryonic  disk 
other  than  that  associated  witli  the  formation  of  the  prim- 
itive streak.  In  an  embr\'o  described  by  Kternod  and 
measuring  1.3  mm.  in  length  (Fig.  54)  a  median  longitu- 
dinal groove  (w)  has  made  its  appearance,  marking 
out  the  axis  of  the  disk  and  forming  what  is  known  as  the 
medullary  ijroove;  and  in  the  older  embryo  described  by 
Spee  (Fig.  38)  a  longitudinal  ridge  has  appeared  on  either 
side  of  the  groove,  forming  the  medullary  folds. 

The  two  folds  are  continuous  anteriorly,  but  behind 
they  are  at  first  separate,  the  anterior  portion  of  the  primi- 
tive streak  lying  between  them.     In  forms,  such  as  the 
Reptilia,  which  possejs  a  distinct  blastopore,  this  opening 
lies  in  the  interval  between  the  two,  and  consequently  is  in 
the  floor  of  the  medullary  groove,  and  in  the  mammalia, 
even  though  no  well-defined  blastopore  is  formed,  yet  at 
the  time  of  the  formation  of  the  medullary  fold  an  opening 
breaks  through  at  the  anterior  end  of  the  primitive  streak 
and  places  the  cavity  lying  below  the  endoderm  in  com- 
munication with  the  space  bounded  by  the  medullary 
folds.     The  canal  so  formed  is  termed  the  neur enteric 
canal  (Fig.  55,  nc)  and  is  so  -ailed  because  it  unites  what 
will  later  become  the  central  canal  of  the  nervous  system 
with  the  intestine  (enteron).      The  significance  of  this 

112 


Fk;.  54. -    Hmbryo  1.,U  mm.  Lo.nc. 
al,  Allantois;  Am,  amnion,  hs,  belly -stalk;  //,  heart;  m,  medullary  j;r(M>ve; 
H.f,    neurenteric   canal;    fy.c,    caudal    protuberance;    ps,    primitive 
streak;  ys,  yolk-stalk. — {Etcrnod.) 


lo 


"3 


M       il 


ii  i 


4  ■     : 


'■4  THK    DKVKLOPMKNT   OK   TlIK    Hl'MAN    fionv. 

canal  is  somewhat  obscure,  and  it  is  of  very  brief  persist- 
ence, closing  at  an  early  stage  of  development  so  as  to 
leave  no  trace  of  its  existence. 

As  development  proceeds  the  medullary  folds  increase 
in  height  and  at  the  same  time  incline  toward  one  mother 


I'u..  55.^D..».;kam  of  a  I,on<;,t,  i.i.vAi.  Section  through  an  Kmbrvo 

OK    1.54    MM. 

al.  Allantois;  ,iw.    amnion;    H,    l.elly-stalk ;   ,/,,    chorion-   //     la-irt  •    «r 
neurentenc  canal;   V,  chorionic  villi;   J.  yolk-s;ic.     (ri;  i>a  )' 

(Fig.  40)  so  that  their  edges  finally  come  into  contact  and 
later  fuse,  the  two  ectodermal  layers  forming  the  one 
imituig  with  the  corresponding  layers  of  the  other  (Fig. 
56).     By  this  process  the  medullary  groove  becomes  con 
verted  into  a  medullary  canal  which  later  becomes  the 


1:1  . 


THE    MEDULLARY   CANAL. 


US 


I 


central  canal  of  the  spinal  cord  and  the  ventricles  of  the 
brain,  the  ectodermal  walls  of  the  canal  thickening  to  give 
rise  to  the  central  nervous  system.  The  closure  of  the 
groove  does  not,  however,  take  place  simultaneously 
along  its  entire  length,  but  begins  in  what  corresponds  to 
the  neck  region  of  the  adult  (K.i;.  41)  and  thence  proceeds 
both  anteriorly  and  posteriorly,  the  extension  of  the  fusion 
taking  place  rather  slowly,  however,  especially  anteriorly, 
so  that  an  anterior  open- 
ing into  the  otherwise 
closed  canal  can  be  dis- 
tinguished for  a  consider- 
able period  (Fig.  42). 

While  these  changes 
liave  been  taking  place 
in  the  ectoderm  .A  the 
■nedian  line  of  the  em- 
bryonic disk,  modifica- 
tions of  the  subjacent  en- 
doderm  have  also  oc- 
curred. This  endoderm, 
it  will  be  remembered, 
was  formed  by  the  head 
process  of  the  primitive 
streak,  and  was  a  plate 
of    cells    continuous    at 

the  sides  with  the  primary  endoderm  and  extending 
forward  as  far  as  what  will  eventually  be  the  anterior 
part  of  the  pharynx.  Along  the  line  of  its  jiuiction  with 
the  primary  endoderm  it  gives  rise  to  the  plates  of  gastral 
mesoderm  (Fig.  27),  while  the  remainder  of  it  produces 
an  important  embryonic  organ  known  as  the  notochord 
or  chorda  dorsalis  and  on  this  account  is  sometuncs 
termed  the  chorda  endoderm. 


Fu;.  56.— Diagrams  sHowiNt;  tub 
Manner  of  the  CuosruE  ok  the 
MEnruLARY  Groove. 


ii6 


THE    DEVELOPMENT   OF   THE    HUMAN    BODY, 


After  the  separation  of  the  plates  of  gastral  mesoderm 
the  chorda  endoderm,  which  is  at  first  a  flat  band, 
becomes  somewhat  curved  (Fig.  57,  A),  so  that  it  is 
concave  on  its  under  surface,  and,  the  curvature  increas- 
ing, the  edges  of  the  plate  come  into  contact  and  finally 
fuse  together  (Fig.  57,  B),  the  edges  of  the  primary  endo- 
derm at  the  same  time  uniting  beneath  the  chofdal  tube 
so  formed,  so  this  layer  becomes  a  continuous  sheet,  as  it 
was  at  its  first  appearance.     The  lumen  which  is  at  first 


ii  i\ 


Fig.    57. — Tr.ansverse   Sections   throigh   Mole    Kmbryos,   SHowiNf. 

THE  Formation;  ok  the  Notochord. 

ec,  r:ctoderm;  en,  endoderm;  m,  mesoderm ^«f,  notochord.— (//fu/je.) 


til  'I 
i  i  * 


1 


present  in  the  chordal  tube  is  soon  obliterated  by  the  en- 
largement of  the  cells  which  bound  it,  and  these  cells  later 
undergo  a  peculiar  transformation  whereby  the  chordal 
t  Ms  converted  into  a  solid  elastic  rod  surrounded  by  a 
cuticular  sheath  secreted  by  the  cells.  The  notochord  lies 
at  first  immediately  beneath  the  median  line  of  the  med- 
ullary groove,  between  the  ectoderm  and  the  endoderm, 
and  has  on  either  side  of  it  the  mesodermal  plates.  It  is  a 
temporary  structure  of  which  only  rudiments  persist  in 


THE    MESODERMIC    SOMITES. 


117 


f 


M 


the  adult  condition  in  man,  but  it  is  a  structure  character- 
istic of  all  vertebrate  embryos  and  persists  to  a  more  or 
less  perfect  extent  in  many  of  the  fishes,  being  indeed  the 
only  axial  skeleton  possessed  by  Amphioxus.  In  the 
higher  vertebrates  it  is  almost  completely  replaced  by  the 
vertebral  column,  which  develops  around  it  in  a  manner 
to  be  described  later. 

Turning  now  to  the  middle  germinal  layer,  it  will  be 
found  that  in  it  also  important  changes  take  place  during 
these  early  stages  of  development.     The  probable  mode 
of  development  of  the  extra-embryonic  mesoderm  and 
body-cavityhas  already  been  described  (p.  84)  and  atten- 
tion may  now  be  directed  toward  what  occurs  in  the  em- 
bryonic mesoderm.     In  both  the  Peters  embryo  and  the 
embryo  v.H  described  by  von  Spec  this  portion  of   the 
mesoderm  is  represented  by  a  plate  of  cells  lying  between 
the  ectoderm  and  endoderm  and  becoming  continuous  at 
the  edges  of  the  embryonic  area  with  both  the  layer  which 
surrounds  the  yolk-sac  and,  through  the  mesoderm  of  the 
belly-stalk,  with  the  chorionic  mesoderm  (Fig.  35).     It 
seems  probable,  since  there  is  in  these  embryos  no  indica- 
tion as  yet  of  the  formation  of  the  chorda  endoderm,  that 
this  plate  of  mesoderm  corresponds  to  the  prostomial 
mesoderm  of  lower  forms.     In  older  embryos,  such  as  the 
embryo  Gle  of  Graf  Spec  and  the  younger  embryo  de- 
scribed by  Eternod  (Fig.  54),  the  mesoderm  no  longer 
forms  a  continuous  sheet  extending  completely  across  the 
embryonic  disk,  but  is  divided  into  two  lateral  plates,  in 
the  interval  between  which  the  ectoderm  of  the  floor  of  the 
medullary  groove  and  the  chorda  endoderm  are  in  close 
contact   (Fig.   34).     These  lateral   plates  represent   the 
gastral  mesoderm,  whose  origin  has  already  been  described 
(p.    77),    and    which   apparently   supplants  the  original 
prostomial  mesoderm,  whose  fate  in  the  human  embryo  is 


Ii8 


THE    DEVELOPMENT    OF    THE    HUMAN    HOnV. 


X 


at  present  unknown.  The  changes  which  now  occur  have 
not  as  yet  been  observed  in  the  human  embryo,  though 
they  probably  resemble  those  described  in  other  mamma- 
lian embryos,  and  the  phenomena  which  occur  in  the  sheep 
may  serve  to  illustrate  their  probable  nature, 
k  The  lateral  plates  increase  in  size  by  the  multiplication 
of  the  cells  which  compose  them  and,  in  sections,  have  a 
somewhat  triangular  form,  the  portions  nearest  the  me- 
dian line  of  the  embryo  being  much  thicker  than  the  more 
lateral  parts.     In  the  region  which  will  later  become  the 


Fig.    58— Transverse   Sectiom   throigh   the   Second    Mesodermic 

Somite  of  a  Sheep  Embryo  3  mm.  Long. 
dm,  Amnion;   en,  endoderm;    /,  intermediate   cell-mass;    m^,  medullary 

groove;   ms,  mesodermic    somite;   so,    somatic    and    />,     splanchnic 

layers  of  the  ventral  mesoderm.  -  {lionnct.) 

neck  of  the  embryo  a  longitudinal  groove  appears  upon 
the  dorsal  surface  of  each  plate,  marking  off  the  more 
median  thicker  portion  from  the  lateral  parts,  and  the 
median  portions  then  become  divided  transversely  into  a 
number  of  more  or  less  cubical  masses  which  are  termed 
the  protovertebrce  or,  better,  mesodermic  somites  (Fig.  58, 
ms),  structures  whose  appearance  in  surface  views  has 
already  been  described  (Figs.  41  et  seq.).  The  cells  of  the 
somites  and  of  the  lateral  parts,  which  may  be  termed  the 


i\^ 


THE    MESODERMIC    SOMITES. 


119 


ventral  mesoderm,  are  at  first  stellate  in  form,  but  later 
become  more  spindle-shaped,  and  those  near  the  center  of 
each  somite  and  those  of  the  ventral  mesoderm  arrange 
themselves  in  regular  layers  so  as  to  enclose  cavities  which 
appear  in  these  regions  (Fig.  58).  The  cavities  of  the 
somites  first  formed  become  continuous  with  the  cavities 
contained  between  the  layers  of  the  adjacent  ventral 
mesoderm,  but  this  continuity  eventually  disappears  and 


''^>. 


Fu;.  59.— Transverse  Section*  of  an  Embryo  of  2.5  .mm.  (See  I-ir..  42) 

SHOWINC  ON  EITHER  SIDE  OF  THE  MEDILUARY  CaNAL  A   MESODERMIC 

Somite,  the  Intermeoiate  Ceu.  mass,  and  the  Ventral  Meso- 
derm.—  (tpm  Lculiossck.) 

is  not  developed  in  the  later  formed  somites.  Each  origi- 
nal lateral  plate  of  gastral  mesoderm  then  becomes  divided 
longitudinally  into  three  areas,  a  more  median  trea  com- 
posed of  mesodermic  somites,  lateral  to  this  a  narrow 
area  underlying  the  original  longitudinal  groove  which 
separated  the  somite  area  from  the  ventral  mesoderm  and 
which  from  its  position  is  termed  the  intermediate  cell 


f    I 


m 


ill 


120 


THE    DEVELOPMENT    OF    THE    HUMAN    UODV. 


mass  (Fig.  58,  i),  and,  finally,  the  ventral  mesoderm. 
This  last  portion  is  now  divided  into  two  layers,  the  dorsal 
of  which  is  termed  the  somatic  mesoderm,  while  the  ventral 
one  is  known  as  the  splanchnic  mesoderm  (Fig.  58,  so  and 
sp;  and  Fig.  59),  the  cavity  which  separates  these  two 
layers  being  the  embryonic  body-cavity  or  pleuroperito- 
neal  cavity,  which  will  eventually  give  rise  to  the  pleural 
pericardial  and  peritoneal  cavities  of  the  adult  as  well  as 
the  cavity  of  each  tunica  vaginalis  testis. 

Beginning  in  the  neck  region,  th(  ormation  of  the 
mescdermic  somites  proceeds  anteriorly  and  posteriorly 
until  finally  there  are  present  in  the  human  embryo  thirty- 
eight  pairs  in  the  neck  and  trunk  regions  of  the  body,  and, 
in  addition,  a  certain  number  are  developed  in  what  is 
later  the  occipital  region  of  the  head.  Exactly  how  many 
of  these  occipital  somites  are  developed  is  not  known,  but 
in  the  cow  four  have  been  observed,  and  there  are  reasons 
for  believing  that  the  same  number  occurs  in  the  human 
embryo. 

In  the  lower  vertebrates  a  number  of  cavities  aTanged  in 
pairs  occur  in  the  more  anterior  portions  of  the  head  and  have 
been  homologized  with  mesodermic  somites.  Whether  this 
homology  be  perfectly  correct  or  not,  these  head- cavities,  as 
they  are  termed,  indicate  the  existence  of  a  division  of  the 
head  mesoderm  into  somites,  and  although  practically  nothing 
is  known  as  to  their  existence  in  ..le  human  embryo,  yet, 
from  the  relations  in  which  they  stand  to  the  cranial  nerves 
and  musculature  in  the  lower  forms,  there  is  reason  to  suppose 
that  they  are  not  entirely  unrepresented. 

The  mesodermic  somites  in  the  earliest  human  embryos 
in  which  they  have  been  observed  contain  a  completely 
closed  cavity,  and  this  is  true  of  the  majority  of  the  somites 
in  such  a  form  as  the  sheep.  In  the  four  first-formed 
somites  in  this  species,  however,  as  has  already  been 
stated,  the  somite  cavity  is  at  first  continuous  with  the 


THE    MESODERMIC    SOMITES. 


121 


i 


pleuroperitoneal  cavity  and  only  later  becomes  separated 
off  from  it,  and  in  lower  vertebrates  this  continuity  of  the 
somite  cavities  with  the  general  body-cavity  is  the  rule. 
The  somite  cavities  are  consecjuently  to  be  regarded  as 


Pn, 


A 


,0.— Transverse  Section  o.-  an  Rmbryo  of  4.25  mm.  at  the  Level 
OF  THE  Arm  Rudiment. 
A,  rtxial  mesoderm  of  arm;  Am,  amnion;  il,  inner  lamella  of  myotome; 
M,  riyotome;  me,  splanchnic  mesoderm;  ol,  outer  lamella  of  myo- 
tome ;7'w,  place  of  origin  of  pronephros;  S,  sclerotome;  i"',  defect  in 
wall  of  myotome  due  to  separation  of  the  sclerotome ;  st,  stomach ; 
I'M,  umbilical  vein. — (KoUmann.) 

portions  of  the  general  pleuroperitoneal  cavity  which  have 
secondarily  been  separated  off.  They  are,  however,  of 
but  short  duration  and  early  become  filled  up  by  spindle- 
shaped  cells  derived  from  the  walls  of  the  somites,  which 


I 


^ 


122 


THE    OKVEI-OrMENT    OF    THE    HUMAN    DODV. 


B 


\  I 


themselves  undergo  a  diflcrentiation  into  distinct  por- 
tions. "  The  cells  of  that  portion  of  the  wall  of  each  somite 
which  is  opposite  the  notochord  become  spindle-shaped 
and  grow  iijward  toward  the  median  line  to  surround  the 
notochord  and  central  nervous  system  and  give  rise  event- 
ually to  the  lateral  half  of  the  body  of  a  vertebra  and  the 
corresponding  portion  of  '.  vertebral  arch.  This  portion 
of  the  somite  is  termed  a  sclerotome  (Fig.  60,  s),  and  the 
remaining  part  of  the  medial  wall  forms  a  muscle  plate  or 
myotome  (m)  which  is  destined  to  give  rise  to  a  portion  of 
the  voluntary  musculature  of  the  body,  while  the  outer 
wall  probably  takes  part  in  the  formation  of  the  cutis 
layer  of  the  skin  and  hence  has  been  termed  the  ctitis 
plate,  or  dermatome. 

The  intermediate  cell-mass  in  the  human  embryo,  as  in 
lower  forms,  partakes  of  the  transverse  divi^'ons  which 
separate  the  individual  mesodermic  somites.  From  one 
portion  of  the  tissue  of  most  of  the  somites  (Fig.  60,  pn) 
the  provisional  kidneys  or  Wolffian  bodies  develop,  this 
portion  of  each  mass  being  termed  a  nephrotome,  while  the 
remaining  portion  gives  rise  to  a  mass  of  cells  showing  no 
tendency  to  arrange  themselves  in  definite  layers  and  con- 
stituting that  form  of  mesoderm  which  has  been  termed 
mesenchyme  (see  p.  80).  These  mesenchymatous  masses 
become  converted  into  connective  tissues  and  blood- 
vessels. 

The  ventral  mesoderm  in  the  neck  and  trunk  regions 
never  becomes  divided  transversely  into  segments  corre- 
sponding to  the  mesodermic  somites,  differing  in  this 
respect  from  the  other  portions  of  the  gastral  mesoderm. 
In  the  head,  however,  that  portion  of  the  middle  layer 
which  corresponds  to  the  ventral  mesoderm  of  the  trunk 
does  undergo  a  division  into  segments  in  connection  with 
the  development  of  the  branchial  arches  and  clefts.     A 


I 


THE    VENTRAI,    MESODERM. 


123 


consideration  of  these  segments,  which  are  known  as  the 
branchiomeres,  may  conveniently  be  postponed  until  the 
chapters  dealing  with  the  development  ol  the  cranial 
muscles  and  nerves,  and  in  what  follows  here  attention 
will  be  confined  to  what  occurs  in  the  ventral  mesoderm 
of  the  neck  and  trunk. 

Its  splanchnic  layer  applies  itself  closely  to  the  cndo- 
dermal  digestive  tract  (Fig.  62,  sp),  which  is  constricted 
off  from  the  dorsal  portion  of  the  yolk-sac,  and  becomes 
converted  into  mesenchyme  out  of  which  the  muscular 
coats  of  the  digestive  tract  develop.  The  cells  which  line 
the  pleuroperitoneal  cavity,  however,  retain  their  arraiige- 
ment  in  a  layer  and  form  a  part  of  the  serous  lining  of  the 
peritoneal  and  other  serous  cavities,  the  remainder  of  the 
lining  being  formed  by  the  corresponding  cells  of  the 
somatic  layer ;  and  in  the  abdominal  region  the  superficial 
cells,  situated  near  the  line  where  the  splanchnic  layer 
passes  into  the  somatic,  and  in  close  proximity  to  the 
nephrotome  of  the  intermediate  cell-mass,  become  col- 
umnar \a  shape  and  are  converted  into  reproductive  cells. 

The  somatic  layer,  if  traced  peripherally,  becomes  con- 
tinuous at  the  sides  with  the  layer  of  mesoderm  which  lines 
the  outer  surface  of  the  amnion  (Fig.  f;o)  and  posteriorly 
with  the  mesoderm  of  the  belly-stalk.  That  portion  of 
it  which  lies  within  the  bodv  of  the  embrvo,  in  addition  to 
giving  rise  to  the  serous  lining  of  the  parietal  layer  of  the 
pleuroperitoneum,  becomes  converted  into  mesenchyme, 
whicl  >r  a  considerable  length  of  time  is  clearly  differen- 
tiatect  .iiLo  two  zones,  a  more  compact  dorsal  one  which 
may  be  termed  the  somatic  layer  proper,  and  a  thinner 
more  ventral  vascular  zone  which  is  termed  the  memhrana 
rcunicns  (Fig.  61).  In  the  earlier  stages  the  somatic  layer 
proper  does  not  extend  ventrally  beyond  the  line  which 
passes  through  the  limb  buds  and  it  grows  out  into  these 


wm 


124 


THE    DEVELOPMENT    OF    THE    HUMAN    IIODV. 


i  3 


buds  to  form  an  axial  core  for  tliem  (Fig.  6i ,  Lr),  in  which 
later  the  skeleton  of  the  limb  forms.  The  remainder  of 
the  mesoderm  lining  the  sides  and  ventral  portions  of  the 
body-wall  is  at  first  formed  from  the  membrana  reuniens, 
but  as  development  proceeds  the  somatic  layer  gradually 
extends  more  ventrally  and  displaces,  or,  more  properly 
speaking,  assimilates  into  itself,  the  membrana  reuniens 
until  finally  the  latter  has  completely  disappeared. 

It  is  to  be  noted  that  no  part  of  the  voluntary  muscula- 
ture of  the  lateral  and  ventral  walls  of  the  neck  and  trunk 
is  derived  from  the  somatic  layer,  nor  do  the  muscles  of 
the  limbs  arise  from  the  axial  core  of  mesenchyme  which 
passes  into  them  from  this  layer.*  All  the  voluntary 
muscles  of  the  neck,  trunk,  and  limbs  are  derived  from 
the  myotomes  which  gradually  extend  ventrally  and  send 
out  also  into  the  limbs  prolongations  which  completely  in- 
vest the  axial  mesenchyme,  and  it  is  probable,  also,  that 
the  ribs  are  derived  from  ventral  prolongations  of  the 
sclerotomes.  The  probable  relations  of  the  various  parts 
derived  from  the  gastral  mesoderm  may  be  perceived 
from  the  diagrams  composing  Fig.  6i,  which  represent 
the  conditions  obtaining  in  embryos  of  different  ages. 

The  appearance  of  the  mesodermic  somites  is  an  im- 
portant phenomenon  in  the  development  of  the  embryo, 
since  it  influences  fundamentally  the  future  structure  of 
the  organism.  If  each  pair  of  mesodermic  somites  be 
regarded  as  an  element  and  termed  a  meiamere  or  seg- 
ment, then  it  may  be  said  that  the  body  is  composed  of  a 
series  of  metameres,  each  more  or  less  closely  resembling 
its  fellows,  and  succeeding  one  another  at  regular  inter- 
vals. Each  somite  differentiates,  as  has  been  stated,  into 
a  sclerotome,  a  myotome,  and  a  cutis  plate,  and,  accord- 


*  See  page  2?<  1 . 


METAMERISM. 


125 


ingly,  there  will  primarily  be  as  many  vertebra*,  muscle 
segments,  and  cutis  segments  as  there  are  mesodermic 
somites,  or,  in  other  words,  the  axial  skeleton,  the  volun- 
tary muscles,  and  the  cutis  are  primarily  metameric.  Nor 
is  this  all.  Since  each  metamcre  is  a  distinct  unit,  it  must 
possess  its  own  supply  of  nutrition,  and  hence  the  primary 
arrangement  of  the  blood-vessels  is  also  metameric,   a 


mo 


Fk;.   61.     Diagrams    Ilhstratin(;    the    History   of   the   Oastral, 

.Mesoderm. 

(",  Cutis  plate;  Dm,  dorsal  portion  of  luyolomc;  (h,  genital  ridge :  /,  in- 
testine; l.r,  limb  Inid;  Mr,  nu'inl)rana  reiiniens;  .V,  nervous  system  ; 
Nc,  notocliord ;  .SV,  sclerotome;  So   and  .S"/>,  somatic  and  splanchnic 
mesoderm;  Vm,  ventral  portion  of  myotome;  \Vd,  Wolffian  duct.- 
{Modi/icd  jrom  Kallmann.) 


branch  passing  ofi"  on  either  side  from  the  main  longitu- 
dinal arteries  and  veins  to  each  metamere.  And,  further, 
each  pair  of  muscle  segments  and  the  corresponding  cutis 
plates  receive  their  own  nerves,  so  t\\?*  the  arrangement 
of  the  nerves,  again,  is  distinctly  metameric. 

This  metamerism  is  most  distinct  in  the  neck  and  trui  k 
regions,  and  at  first  only  in  the  dorsal  portions  of  these 


1^6 


THE    DEVELOPMENT   OF    THE    HUMAN    BODY. 


I 


h) 


regions,  the  ventral  portions  showing  metamerism  only 
after  the  extension  into  them  of  the  myotomes.  But  'nere 
is  clear  evidence  that  the  arrangement  extends  also  into 
the  head,  and  that  this,  like  the  rest  of  the  body,  is  to  be 
regarded  as  composed  of  metameres.  It  has  been  seen 
that  in  the  notocliordal  region  of  the  head  of  lower  verte- 
brates mesodermic  somites  are  present,  while  anteriorly 
in  the  prechordal  region  there  are  head-cavities  which 
resemble  the  mesodermic  somites  in  that  tlieir  walls  be- 
come converted  into  muscle  tissue,  and  which  may,  per- 
haps, be  directly  comparable  to  the  somites  of  the  trunk. 
There  is  reason,  therefore,  for  believing  that  the  funda- 
mental arrangement  of  all  parts  of  the  body  is  metameric, 
but  though  this  arrangement  is  clearly  defined  in  early 
embryos,  it  loses  distinctness  in  later  periods  of  develop- 
ment. The  various  cutis  metameres  early  unite,  so  that 
their  primary  relations  become  greatly  obscured,  and  the 
same  is  true  to  a  certain  extent  of  the  muscle  segments 
and  of  the  blood-vessels;  but  even  in  the  adult  the  pri- 
mary metamerism  is  clearly  indicated  in  the  arrangement 
of  the  nerves  and  of  parts  of  the  axial  skeleton,  and  careful 
study  frequently  reveals  indications  of  it  in  highly  modi- 
fied muscles  and  blood-vessels. 

In  the  head  the  development  of  the  branchial  arches 
and  clefts  produces  a  series  of  parts  presenting  many  of 
the  peculiarities  of  metameres,  and,  indeed,  it  has  been 
a  very  general  custom  to  regard  them  as  expressions  of  the 
general  metamerism  which  prevails  throughout  the  body. 
It  is  to  be  noted,  however,  that  they  are  produced  by  tlie 
segmentation  of  the  ventral  mesoderm,  a  structure  which 
in  the  neck  and  trunk  regions  docs  not  share  in  the  general 
metamerism,  and,  furthermore,  recent  observations  on 
tlie  cranial  nerves  seem  to  indicate  that  these  branchio- 
meres  cannot  be  regarded  as  portions  of  the  head  meta- 


LITKRATURE. 


127 


mcrcs  or  even  structures  comparable  to  these.  They 
represent,  mo:c  probably,  a  second  metamerism  super- 
posed upon  the  more  :;;eneral  one,  or,  indeed,  possibly 
more  primitive  than  it,  but  whose  relations  can  only  be 
properly  understood  in  connection  with  a  study  of  the 
cranial  nerves  (see  p.  431  )• 


LITERATURE. 
In  luldition  tn  many  of  tin-  pujHjrs  cited  in  tlie  list  at  tiic  close  of  t."lia|)ter 
II,  the  following  may  he  mentioned: 

W.  Heai'E:  "The  Development  of  the  Mole  (Talpa  ICuropaa),"  tJuorUrly 

Journ.  Microsc.  Science,  xxvii,  1887. 
F.  KeiBEu:  "JCur  Ivntwickelungsgcschichte  der  Chorda  bei  SauRern  (Meer- 

schweinchen  und  Kaninchen),"  Archiv  jiir  Anal,  und  I'liysiul.,  Amit 

Ahth.,   1889. 
J.   Kcjllma.n'n:  "Die   Runipfsegniente  menschlicher  Kmhryonen  von   13 

his  35  rrwirl)eln,"  Archiv  jiir  Anat  und  Physiol.,  Anal.  Ahth.,  1891. 
J.  \V.  v.\.N  WijiiK:  "  Uel)er  die  Mesodertnsegmente  des  Runipfes  und  die 

I'lntvvicklung  des    Kxcretionsystems  hei  .Selachiern,"    Archiv  fur  mi- 

krosk.  Amit.,  xx.xiii,   1889. 
K.    W.    ZimmERMA.vn:   "  Uehcr    Kopfliohlenrudimente  beim  Menschen," 

Archiv  jiir  mikrosk.  Anal.,  uiii,   1898. 


41 


i 


it 


CHAITKR  V. 

THE  YOLK-STALK.  BELLY-STALK,  AND  FETAL 

MEMBRANES. 

The  conditions  to  which  the  embryos  and  larva*  of  the 
majority  of  animals  must  adapt  themselves  are  so  differ- 
ent from  those  under  which  the  adult  organisms  exist 
that  in  the  early  stages  of  development  special  organs  are 
very  frequently  developed  which  are  of  use  only  during 
the  embryonic  or  larval  period  and  are  discarded  when 
more  advanced  stages  of  development  have  been  reached. 
This  remark  applies  with  especial  force  to  the  human  em- 
bryo which  leads  for  a  period  of  nine  months  what  may 
be  termed  a  parasitic  existence,  drawing  its  nutrition  from 
and  yielding  up  its  waste  products  to  the  blood  of  the 
parent.  In  order  that  this  may  be  accomplished  certain 
special  organs  are  developed  by  the  embryo,  by  means  of 
which  it  forms  an  intimate  connection  with  the  walls  of 
the  uterus,  which,  on  its  part,  becomes  greatly  modified, 
the  combination  of  embryonic  and  maternal  structures 
producing  what  is  termed  the  dcciducr,  owing  to  its  being 
discarded  at  birth  when  the  parasitic  mode  of  life  is  given 
up. 

Furthermore,  it  has  already  been  seen  that  many  pecu- 
liar modifications  of  development  in  the  human  embryo 
result  from  the  inheritance  of  structures  from  more  or  less 
remote  ancestors,  and  among  the  embryonic  adnexes  are 
found  structures  which  represent  in  a  more  or  less  modi- 
fied condition  organs  of  considerable  functional  impor- 
tance in  lower  forms.     Such  structures  are  the  yolk-stalk 

128 


THE   AMNION. 


129 


and  vesicle,  the  amnion,  and  the  aUantois,  and  for  their 
proper  understanding  it  will  be  well  to  consider  briefly 
their  development  in  some  lower  form,  such  as  the  chick. 
At  the  time  when  the  embryo  of  the  chick  begins  to  be 
constricted  off  from  the  surface  of  the  large  yolk-mass,  a 
fold,  consisting  of  ^ndrrm  nnd  soutatic  mesoderm,  arises 
iust  outside  thej-mhrynr'"  ----  ^^'^^^  it  completely  sur- 


Fu;.   62.- 


I)IA(;R.\MS     ItLlSTRATIN.;     THE     FORMATION     OK     THE     AmNKN 
AND  AlUANTOIS  IN  THE  ChICK. 

,U     Atnni.m    folds;   .4/.    aUantois;   Am,   amniotic   cavity;    />s,    yolk-sac. 

—  (^Gtginhaur.) 


rounds.  As  development  proceeds  the  fold  becomes 
higher  and  its  edges  gradually  draw  nearer  togethr;  over 
the  dorsal  surface  of  the  embryo  (Fig.  62,  A),  and  finally 
meet  and  fuse  (Fig.  62,  B),  so  that  the  embryo  becomes 
enclosed  within  a  sac,  which  is  leiined  the  amnion  and  is 
formed  by  the  fusion  of  the  layers  which  constituted  the 
inner  wall  of  the  fold.     The  layers  of  the  outer  wall  c.i  the 


II 


jsam 


I30 


THE 


DEVELOPMENT    OF   THE    HUMAN    BODY. 


fold  after  fusion  form  part  of  the  general  ectoderm  and 
somatic  mesoderm  which  make  up  the  outer  wall  of  the 
ovum  and  together  are  known  as  the  serosa,  corresponding 
to  thejchorion  of  the  mammalian  embryo.  The  space 
which  occurs  between  the  amnion  and  the  serosa  is  a  por- 
tion of  the  extra-embryonic  coelom  and  is  continuous  with 
the  embryonic  pleuroperitoneal  cavity. 

In  the  ovum  of  the  chick,  as  in  that  of  the  reptile  the 
protoplasmic  material  is  limited  to  one  pole  and  rests  upon 
the  large  yolk-mass.     As  development  proceeds  the  germ 
layers  gradually  extend  around  the  yolk-mass  (compare 
tig.  62,  A-C)  and  eventually  completely  enclose  it   the 
yolk-mass  coming  to  lie  within  the  endodermal  layer 
which,  together  with  the  splanchnic    mesoderm  which 
lines  It,  forms  what  is  termed  the  volk-sac.     As  the  em- 
bryo separates  from  the  yolk-mass  the  volk-sac  is  con- 
stricted in  Its  proximal  portion  and  so  differentiated  into 
a  yolk-stalk  and  a  yolk-sac,  the  contents  of  the  latter  being 
gradually  absorbed  by  the  embryo  during  its  growth   its 
walls  and  those  of  the  stalk  being  converted  into  a  portion 
of  the  embryonic  digestive  tract. 

In  the  mean  time,  however,  from  the  posterior  portion 
of  the  digestive  tract,  behind  the  point  of  attachment  of 
A?  ^^^■^^'''  ^  diverticulum  has  begun  to  form  (Fig   6- 
A)      This  increases  in  size,  projecting  into  the  extra- 
embryonic portion  of  the  pleuroperitoneal  cavity  and 
pushing  before  it  the  splanchnic  mesoderm  which  lino's  the 
endoderm  (Fig.  62,  B  and  C).    This  is  the  allantois,  which 
reaching  a  very  considerable  size  in  the  chick,  and  applv- 
mg  Itself  closely  to  the  inside  of  the  serosa,  serves  as  a 
respiratory  and  excretory  organ  for  the  embryo,  for  which 
purpose  Its  walls  are  richly  supplied  with  blood-vessels 
the  allantoic  arteries  and  veins. 

Toward  the  end  of  the  incubation  period  l)oth  the  am- 

\ 


THE    AMNION. 


-131- 


nion  and  allantois  begin  to  undergo  retrogn  sive  changes, 
and  just  before  the  hatching  of  the  young  chick  tliey  be- 
come completely  dried  up  and  closely  adherent  to  the 
egg-shell,  at  the  same  time  separating  from  their  point  of 
attachment  to  the  body  of  the  young  chick,  so  that  when 
the  chick  leaves  the  egg-shell  it  bursts  through  thedried- 
up  membranes  and  leaves  them  behind  as  useless  struc- 
tures. 

The  Amnion. — Turning  now  to  the  human  embryo,  it 
will  be  found  that  the  same  organs  are  present,  though 
somewhat  modified  either  in  the  mode  or  the  extent  of 
their   development.     A   well-developed   amnion   occurs, 
arising,  however,  in  a  very  different  manner  from  what  it 
does  in  the  chick ;  a  Inr^r^  volk-snc^  oc^riirs  even  thoudi  it 
contains  no  yolk ;  and  an  allantois  whjch  has  no  respira- 
tory  or  excretory  functions  is  present,  though  in  a  some- 
what degenerated  condition^    It  has  been  seen  from  the 
description  of  the  earliest  stages  of  development  that  the 
processes  which  occur  in  the  lower  forms  are  greatly  abbre- 
viated in  the  human  embryo.     The  enveloping  layer, 
instead  of  gradually  extending  from  one  pole  to  enclose 
the  entire  ovum,  develops  in  situ  during  the  stages  imme- 
diately succeeding  segmentation,  and  the  extra-embry- 
onic mesoderm,  instead  (^f  growing  out  from_tlie_em^ryo 
to  enclose  the  yglk-sac.  splits  off  dire,ctb^  f|;omJJie,£Uvel- 
oping  layer.     The  earliest  stages  in  the  development  of 
the  amnion  are  not  yet  known  for  the  human  embryo,  but 
from  the  condition  in  which  it  is  found  in  the    Peters 
embryo   (Fig.  35)   and  in  the  embryo  v.H.  of  von  Spec 
(Fig.  37)  it  is  probable  that  it  arises,  not  by  the  fusion  of 
the  edges  of  a  fold,  as  in  the  chick,  but  by  a  vacuolization 
of  a  portion  of  the  innc.  cell-mass,  as  has  been  described 
as   occurring  in  the  bat  (p.  71).     It    is,  then,  a  closed 
cavity  from  the  very  beginning,  the  floor  of  the  cavity 


i 


i  i 


II; 


132  THE    DEVELOPMENT   OF    THE    HUMAN    BODV. 

beins  formed  by  the  embryonic  disk,  its  posterior  wall  by 
the  anterior  surface  of  the  belly-stalk,  while  its  roof  and 
sides  are  thin  and  composed  of  a  sinjjle  layer  of  flattened 
ectodermal  cells  lined  on  the  outside  by  a  laver  of  meso- 


Fr.;.  6,V~  D,..,;k.vms  Itu  sTRAT.N-r.  THE  KoRM.xTtfr^nSlTTHE  Umbiucac 

Lord. 
The  heavy  black  line  represents  the  embryonic  ectoderm-    the  H„tt^rl 
he  am'Z""''  1  "  f^  "  ••^"^■^*'.'"  ^'  »"'  '->y  ^'^t-"""  i"  o  t1  at  0I 
onic  cTl  n'-  /;]  ■.ir'"r  I'  '^r'V  •■  '•  ^'"''"^"^^^  ^''■'  extra-embry. 

.    -d;T;'ch:;ri!;nic':iiiir?-;:^3^:!k^r""^ ' ■  '^■--^'^^  ^-^^  """^•'-' 

derm  continuous  with  the  somatic  mesoderm  of  the  em- 
bryo and  the  mesoderm  of  the  belly-stalk  (Fig  6^  A) 

VVhen  the  bending  downward  of  the  peripheral  portions 
of  the  embryonic  disk  to  close  in  the  ventral  surface  of  the 


THE    AMNION. 


133 


embryo  occurs,  the  line  of  attachment  of  the  amnion  to 
the  disk  is  also  carried  ventrally  (Fig.  63,  B),  so  that  when 
the  constriction  off  of  the  embryo  is  practically  completed, 
the  amnion  is  attached  anteriorly  to  the  margin  of  the 
umbilicus  and  posteriorly  to  the  extremity  of  the  band  of 
ectoderm  lining  what  may  now  be  considered  the  posterior 
surface  of  the  belly-stalk,  while  at  the  sides  it  is  attached 
along  an  oblique  line  joining  these  two  points  (Figs.  63,  B 
and  C,  in  which  the  attachment  of  the  amnion  is  indicated 
by  the  broken  line). 

Leav'^'^  aside  for  the  present  the  changes  which  occur 
in  the  'iment  of  the  amnion  to  the  embryo  (see  p. 

139),  it  '  .,j  be  said  that  during  the  later  growth  of  the 
embryo  the  amniotjcLcavity  increase^  in  jize  untjljinally 
its  wall  comes  into  rnj^jjirf  witjijjie  chorion,  ithe  extra- 
embryonic body-cavity  being  thus  practically  obliterated 
(Fig.  63,  D),  though  jio^^clualJ]4sion_o^f  a^n^n  §^  cho- 
rion occurs.  Suspended  by  the  umbilical  cord,  which  has 
by  this  time  deY.eloped,  the  embryo  floats  freely  in  the 
amniotic  cavity,  which  is  filled  by  a  fluid,  the  liquor 
amnii,  whose  origiij_is_inv^ved_iu  doubt,  some  authors 
maintaining  that  it  infiltrates  into  the  cavity  from  the 
maternal  tissues,  while  others  hold  that  a  certain  amount 
of  it  at  least  is  derived  from  the  embryo.  It  is  a  fluid  with 
a  specific  gravity  of  about  i  .003  and  contains  about  i  per 
cent,  of  solids,  principally  albumin,  grape-sugar,  and  urea, 
the  last  constituent  probably  coming  from  the  embryo. 
When  preset  in  greatest  qi:antity, — that  is  to  say,  at 
about  the  beginning  of  the  last  month  of  pregnancy, — it 
varies  in  amount  between  J  and  ^  of  a  liter,  but  during  the 
last  month  it  diminishes  to  about  half  that  quantity.  To 
protect  the  epidermis  of  the  fetus  from  maceration  during 
its  prolonged  immersion  in  the  liquor  amnii,  the  sebaceous 
glands  of  the  skin  at  about  the  sixth  month  of  develop- 


ili 


134 


THE    DEVELOPMENT   OF     TIIE    HUMAN    BODY. 


ment  pour  out  upon  the  surface  of  the  body  a  white  fatty 
secretion  known  as  the  vernix  caseosa. 

During  parturition  the  amnion,  as  a  rule,  ruptures  as 
the  result  of  the  contraction  of  the  uterine  walls  and  the 
liquor  amnii  escapes  as  the  "waters,"  a  phenomenon 
whieli  normally  precedes  the  delivery  of  the  child.  As  a 
rui'-  .ne  rupture  is  sufficiently  extensive  to  allow  the  pas- 
sage of  the  child,  the  amnion  remaining  behind  in  the 
uterus,  to  be  subsequently  expelled  along  with  the  de- 
ciduae. 

Occasionally  it  happens,  however,  that  the  amnion  is  suffi- 
ciently strong  to  withstand  the  pressure  exerted  upon  it  by 
the  uterine  contractions  and  the  child  is  born  still  enveloped 
in  the  amnion,  which,  in  such  cases,  is  popularly  known  as 
the  "caul,"  th^-  possession  of  which,  according  to  an  old  super- 
stition, marks  the  child  as  a  favorite  of  fortune. 

As  stated  above,  the  liquor  amnii  varies  considerablv  in 
amount  in  different  cases,  and  occasionallv  it  mav  be  present 
in  excessive  quantities,  producing  a  condition  known  as 
hydramnios.  On  the  other  hand,  the  amount  mav  fall  con- 
siderably below  the  normal,  in  which  case  the  amnion  may 
form  abnormal  unions  with  the  embrvo,  sometimes  producing 
malformations.  Occas'onally  also  bands  of  a  fibrous  char- 
acter traverse  the  amniotic  cavity  and,  tightening  upon  the 
embryo  during  its  growth,  may  produce  various  malformations, 
such  as  scars,  splitting  of  the  evelids  or  lips,  or  even  amputa- 
tion of  a  limb. 

The  Yolk=sac. — The  development  of  the  yolk-sac  in  the 
human  embryo,  its  differentiation  into  yolk-stalk  and 
yolk-vesicle,  and  its  enclosure  within  the  i  mbilical  cord 
have  already  been  described.  When  these  changes  have 
been  completed,  the  vesicle  is  a  small  pyriform  structure 
lying  between  the  amnion  and  the  chorionic  mesoderm, 
some  distance  away  from  the  extremity  of  the  umbilical 
cord  (Fig.  63,  D),  and  the  stalk  is  a  long  slender  column 
of  cells  extending  from  the  vesicle  through  the  umbilical 
cord  to  unite  with  the  intestinal  tract  of  the  embryo.    The 


THE    YOLK-SAC. 


135 


vesicle  persists  until  birth  and  may  be  found  among  the 
decidual  tissues  as  a  small  sac  measuring  from  3  to  10  mm. 
in  its  longest  diameter.  The  stalk,  liowevc.  early  under- 
goes degeneration,  the  lumen  which  it  at  first  contains 
becoming  obliterated  and  its  endoderm  also  disappearing 
as  early  as  the  end  of  the  second  month  of  development. 
The  portion  of  the  stalk  which  extends  from  the  umbilicus 
to  the  intestine  usualh'  shares  in  the  degeneration  and  dis- 
appears, but  in  about  3  per  cent,  of  cases  it  persists,  form- 
ing a  more  or  less  extensive  diverticulum  of  the  lower  part 
of  the  small  intestine,  sometimes  only  half  an  inch  or  so 
in  length  and  sometimei,  much  larger.  It  may  or  may 
not  retain  connection  with  the  abdominal  wall  at  the 
umbilicus,  and  is  known  as  Meckel's  diverticulum. 

This  embryonic  rudiment  is  of  no  little  importance,  since, 
when  present,  it  is  apt  to  undergo  invagination  into  the  lumen 
of  the  small  intestine  and  so  occlude  it.  How  frequently' 
this  happens  relatively  to  the  occurrence  of  the  diverticulum 
may  be  judged  from  the  fact  that  out  of  100  cases  of  occlusion 
of  the  small  intestine  6  were  due  to  an  invagination  of  the 
diverticulum. 

In  the  reptiles  and  birds  the  yolk-sac  is  abundantly 
supplied  with  blood-vessels  by  means  of  which  the  absorp- 
tion of  the  yolk  is  carried  on,  and  even  although  the  func- 
tional importance  of  the  yolk-sac  as  an  organ  of  nutrition 
is  almost  nil  in  the  human  embryo,  yet  it  still  retains  a 
well-developed  blood-supply,  the  walls  of  the  vesicle  espe- 
cially possessing  a  rich  network  of  vessels.  The  future 
history  o^  these  vessels,  which  are  known  as  the  omphalo- 
mesente>ic  vessels,  will  be  described  later  on. 

The  Allantois  and  Bel!y=stalk. — It  has  been  seen  that  in 
reptilian  and  avian  embryos  the  allantois  reaches  a  high 
degree  of  development  and  functions  as  a  respiratory  and 
excretory  organ  by  coming  into  contact  with  what  is 
comparable  to  the  chorion  of  the  mammalian  embryo. 


iii 


I 


136 


THE    DEVELOPMENT    OF    THE    HUMAN    IIODY. 


In  man  it  subserves  similar  functions,  but  is  very  much 
modified  both  in  its  mode  of  development  and  in  its  rela- 
tions to  other  parts,  so  that  its  resemblance  to  the  avian 
organ  is  somewhat  obscured.     The  differences  depend 
partly  upon  the  remarkable  abbreviation  manifested  in 
the  early  development  of  the  htiinan  embryo  and  partly 
upon    the  fact   that    the  allautois  serves  to   place   the 
embryo   in    relation  with   the   maternal  blood,   instead 
of  with  the  external  atmosphere,  as  is  the  case  in  the 
egg-laying    forms.      Thus,   the    endodermal    portion    of 
the  allantois,  instead  of  arising  from  the  intestine  and 
pushing  before  it  a  layer  of  splanchnic  mesoderm  to  form  a 
large  sac  lying  freely  in  the  extraembryonic  portion  of  the 
body-cavity,  appears  in  the  human  embryo  before  the 
intestine  has  differentiated  from  the  yolk-sac  and  pushes 
its  way  into  the  solid  mass  of  mesoderm  which  forms  the 
belly-stalk  (Fig.  6.^,  A).     To  understand  the  significance 
of  this  process  it  is  necessary  to  recall  the  abbreviation  in 
the  human  embryo  of  the  development  of  the  extra-em- 
bryonic mesoderm  and  body-cavity.     Instead  of  growing 
out  from  the  embryonic  area,  as  it  does  in  the  lower  forms, 
this  mesoderm  develops  in  situ  by  splitting  off  from  the 
layer  of  enveloping  cells  and,  furthermore,  the  .  xtra- 
embryonic  body-cavity  arises  by  a  splitting  of  the  meso- 
derm so  formed  before  there  is  any  trace  of  a  splitting  of 
the  embryonic  mesoderm  (Figs.  36  and  35).     The  b'-lly- 
stalk,  whose  development  from  a  portion  of  the  inner  cell- 
mass  has  already  been  traced  (p.  85),  is  to  be  regarded  as 
a  portion  of  the  body  of  the  embryo,  since  the  ectoderm 
which  covers  one  surface  of  it  resembles  exactly  that  of  the 
embryonic  disk  and  shows  an  extension  backward  of  the 
medullary  groove  upon  its  surface  (Fig.  64).     The  meso- 
derm, therefore,  of  the  belly-stalk  is  to  be  regarded  as  a 
portion  of  the  embryonic  mesoderm  which  has  not  yet 


THE    AI.I.ANTOIS. 


137 


1 


undergone  a  splitting  into  somatic  and  splanchnic  layers, 
and,  indeed,  it  never  does  undergo  t^ucli  a  splitting,  so  that 
there  is  no  body-cavity  into  which  the  endodermal  allan- 
toic diverticulum  can  grow. 

But  this  does  not  account  for  all  the  peculiarities  of  the 
human  allantois.  In  the  birds,  and  indeed  in  the  lower 
oviparous  mammals,  the  endodermal  portion  of  the  allan- 
tois is  equally  developed  with  the  mesodermal  portion, 
the  allantois  being  an  extensive  sac  whose  cavity  is  filled 
.with  fluid,  and  this  is  also  true  of  such  mammals  as  the 
marsupials,  the  rabbit,  and  the 
ruminants.  In  man,  however, 
the  endodermal  diverticulum 
never  becomes  a  sac-like  struc- 
ture, but  is  a  slender  tube  ex- 
tending from  the  intestine  to  the 
chorion  and  lying  in  the  sub- 
stance of  the  mesoderm  of  the 
belly-stalk  (Fig.  63,  D),  the 
greater  portion  of  which  is  to 
be  regarded  as  homologous  with 
the  relatively  thin  layer  of 
splanchnic  mesoderm  covering 
the  endodermal  divertic'ilum  of 
the  chick.     An  explanation  of 

this  disparity  in  the  development  of  the  mesodermal  and 
endodermal  portions  of  the  human  allantois  is  perhaps 
to  be  found  in  the  altered  conditions  under  which  the  res- 
piration and  secretion  take  place.  In  all  forms,  the  lower 
as  well  as  the  higher,  it  is  the  mesoderm  which  is  the  more 
important  constituent  of  the  allantois,since  in  it  the  blood- 
vessels, upon  whose  presence  the  physiological  functions 
depend,  arise  and  are  embedded.  In  the  birds  and  ovip- 
arous mammals  there  are  no  means  by  which  excreted 


Kic.  64.— Tr.\nsverse  Sec- 
tion THROUGH  THE  BELLY- 
STAUK  OF  AN  EmBRYO  OK 
2.15   MM. 

Aa,  Umbilical  (allantoic)  ar- 
tery; All,  allantois;  'am, 
amnion;  Va,  umbilical  (al- 
lantoic) vein.  —{His.) 


IS 


i  I 


.'  .1 


'■'i 


t , 


I -.8 


THE    HEVELOl'MENr    OK     THE    HUMAN    BODV.     " 


material  can  be  passed  to  the  exterior  of  the  ovum,  an  1  it 
is,  therefore,  stored  up  within  the  cavity  of  the  allantois, 
the  allantoic  fluid  containing  considerable  quantities  of 
nitrogen,  indicating  the  presence  of  urea.     In  the  higher 
mammals  the  intimate  relations  which  develop  between 
the  chorion  and  the  uterine  walls  allow  of  the  passage  of 
excreted  fluids  into  the  maternal  blood;  and  the  more 
intimate  these  relations,  the  less  necessity  there  is  for  an 
allantoic  cavity  in  which  c      reted  fluid  may  be  stored  up. 
The  difference  in  the  development  of  the  cavity  in  the 
ruminants,  for  example,  and  man  depends  probably  upon 
the  greater  intimacy  of  the  union  between  ovum  and 
uterus  in  the  latter,  the  arrangement  for  the  passage  of 
the  excreted  material  into  the  maternal  blood  being  so 
perfect  that  there  i^  practically  no  need  for  the  develop- 
ment of  an  allantt;,,  cavitv. 

The  portion  of  the  endodermal  diverticulum  which  is 
enclosed  within  the  umbilical  cord  persists  until  birth  in  a 
more  or  less  rudimentary  condition,  but  the  intra-embry- 
onic  portions  of  the  allantois  reach  a  greater  development, 
the  more  proximal  portions  acquiring  a  cavity  of  consid- 
erable extent  and  forming  the  urogenital  sinus  and  the 
urinary  bladder,  while  the  portion  intervening  between 
the  apex  of  the  bladder  and  the  umbilicus  becomes  con- 
verted into  a  solid  cord  of  fibrous  tissue  termed  the 
urachus. 

Occasionally  a  lumen  persists  in  the  urachal  portion  of  the 
allantois  and  may  open  to  the  exterior  at  the  umbilicus,  in 
which  case  urine  from  the  bladder  may  escape  at  the  umbilicus. 

Since  the  allantois  in  the  human  embryo,  as  well  as  in 
the  lower  forms,  is  responsible  for  respiration  and  excre- 
tion, its  blood-vessels  are  well  developed.     They  are  repre 
sented  in  the  belly-stalk  by  two  veins  and  two  arteries 
(Fig.  64),  known  in  human  embryology  as  the  umbilical 


THE    UMBILICAL   COKU. 


•39 


veins  and  arteries,  which  extend  from  the  body  of  the 
embryo  out  to  the  chorion,  there  brancliinj^  repeatedly  to 
enter  the  numerous  cliorionic  vilH  by  which  the  embryonic 
tissues  are  phiced  in  rekition  with  the  maternal. 

The  Umbilical  Cord.— During  the  process  of  closing  in 
of  the  ventral  surface  of  the  emljryo  a  stage  is  reached  in 
which  the  embryonic  and  extra-embryonic  portions  of  the 
body  cavity  are  ^.-ompletely  separated  except  for  a  small 
area,  the  umbilicus,  through  which  the  yolk-stalk  passes 
out  CFitf.  64.  B).  At  the  edges  of  this  area  in  front  and  at 
the  sides  tlie  embryonic  ectoderm  and  somatic  mesoderm 
become  continuous  with  the  corresponding  layers  of  the 
amnion,  but  posteriorly  the  line  of  attachment  of  the 
amnion  rvisses  up  upon  the  sides  of  the  belly-stalk  (Fig.  63 
B),  so  that  the  whole  of  the  ventral  surface  of  the  stalk 
is  entirely  uncovered  by  ectoderm,  this  layer  being  limited 
to  its  dorsal  surface  (Fig.  64).  In  subsequent  stages  the 
embryonicj-'ctoclenn  and  somatic  mesoderm  at  the  edges 
of  the  umbilicus  grow  out  ventrally,  carrying  with  them 
the  Hne  of  attacliment  of  the  amnion  and  forming  a  tube 
which  encloses  the  proximal  part  of  the  yolk-stalk.  The 
ectoderm  of  the  belly-stalk  at  the  same  time  extending 
more  laterally,  the  condition  represented  in  Fig.  63,  C,  is 
produced,  and,  these  processes  continuing,  the  entire  belly- 
stalk,  together  with  the  yolk-stalk,  becomes  enclosed 
within  a  cylindrical  cord  extending  from  the  ventral  sur- 
face of  the  body  to  the  chorion  and  forming  the  umbilical 
cord  (Fig.  63,  D). 

From  this  mode  of  development  it  is  evident  that  the 
cord  is,  strictly  speaking,  a  portion  of  the  embryo,  its  sur- 
faces being  completely  covered  by  embryonic  ectoderm, 
the  amnion  being  carried  during  its  formation  further  and 
further  from  the  umbilicus  until  finally  it  is  attached 
around  the  distal  extremity  of  the  cord. 


X 


'(' 


*  h 


>40  THK    DEVELOPMENT    (.1      THE    HUMAN    liODV. 


-UV 


IMBRYOS 


Fig.  65.— Transverse  Sections  of  the  Umbilical  Coro  of  Em 

OK   (/I)     1.8   CM.    AND   {B)    25    CM. 

al,  Allantois;  c.  coelom;   ua,  umbilical   artery;  uv.   umbilical  vein-  xs 

yolk-stalk.  '  •'  ' 


TIIK    UMHir.ICAI.    COKI). 


141 


In  enclosing  the  yolk-stalk  the  umbilical  cor(l_encloses 
■.k^ji_j^ninrLpf>rti()n  of  what  was  orkinallY-the  extra- 
ftnhrvonic  body  cavitv  surrounding  the  yolk-stalk.  A 
section  of  the  cord  injm  early  .stage  of  its  development 
(Fig.  65,  A)  will  sliow  a  thick  mass  of  mesoderm  occupying 
its  dorsal  region;  this  represents  the  mesoderm  of  the 
bellx  stalk  and  contains  the  allantois  and  the  umbilical 
arteries  and  vein  (the  two  veins  originally  present  in  the 
belly-stalk  having  fused),  while  toward  tlie  ventral  sur- 
face there  will  be  seen  a  distinct  cavity  in  which  lies  the 
yolk-stalk  with  its  accompanying  blood-vessels.  The 
portion  of  this  ccelom  nearest  the  body  of  the  embryo  be- 
comes much  enlarged,  and  during  the  second  month  of 
development  contains  some  coils  of  the  small  intestine, 
but  later  the  entire  cavity  becomes  more  and  more  en- 
croached upon  by  the  growth  of  the  mesoderm,  and  at 
about  the  fourth  month  is  entirely  obliierated.  A  section 
of  the  cord  suljsequent  to  that  period  of  development  will 
show  a  solid  mass  of  mesoderm  in  which  are  embedded 
the  umbilical  arteries  and  vein,  the  allantois,  and  the 
rudiments  of  the  yolk-stalk  (Kig.  65,  B). 

When  fully  formed,  thi>  umbilical  cord  measures  on  the 
average  55  cm.  ia  length,  though  it  varies  considerably  in 
diiTerent  cases,  and  has  a  diameter  of  about  1.5  cm.  It 
presents  the  appearance  of  being  spirally  twisted,  an 
appearance  largely  due,  however,  to  the  spiral  course 
pursued  by  the  umbilical  arteries,  though  the  entire  cord 
may  undergo  a  certain  amount  of  torsion  from  the  move- 
ments of  the  embryo  in  the  later  stages  of  development  and 
may  even  be  knotted.  The  greater  part  of  its  substance 
is  formed  by  the  mesoderm,  the  cells  of  which  become 
stellate  and  form  a  reticulum,  the  meshes  of  which  are 
occupied  by  connective-tissue  fibrils  and  a  mucous  fluid 
which  gives  to  the  tissue  a  jelly-like  consistence,  whence 
it  has  received  the  name  of  Wharton's  jelly. 


if 


142 


IMK    OKVEr-OI'MENT   OK     TIIK    lUMAN    ItoDV. 


I 


i  f 


The  Chorion.  -The  umbilical  cord,  or,  more  properly 
the  belly-stalk,  places  the  body  of  the  embrvo  in  commu- 
mcation  with  the  wall  of  the  enil)rv;omc_ves[cle,  and  this 
wall,  termed  the  chaxMui,  is  in  contact  with  the  walls  of  the 
uterus  and  becoifies  specially  modified  to  prod-i-e  the  ^on 
nection  l)etween  the  embryo  and  the  matern.u  tissues 
which  is  characteristic  of  all  the  higher  mammalia  It  is 
composed  of  two  layers,  an  outer  eetuikrnuiJ  tr..plu.l.h,st 
layer  and  an  iiiftcT3;hoi^)nic  nKsudLrm.  In  the  earliest 
sta^a's  it  may  be  presumed  that  the  tropIiol,last   is  com- 


102 


Fig.  66.-T\vo  Diagrams  Illustrating  the  I.\.kmation  of  Chorionic 

Villi. 

ii'osZ    s!    :L"^T'''  -^i-  «^''""-'"i^-   n,cso,lcrm;    v/,,   stratum  sp.,n-' 
giosum,.S>',  syncytium;  I r,  tn.phohlast ;  ;■.  villus. -(Pc/crv.) 

parativcly  thin,  as  in  the  bat's  ovum,  and  later  becomes 
a  stout  layer  many  cells  thick.  In  the  Peters  embryo 
whose  ovum  measures  only  about  i  mm.  in  diameter  it 
has  already  become  quite  thick  and  contains  numerous 
blood  lacunae  arranged  as  a  network  throughout  its  sub- 
stance. These  lacunae  seem  to  have  been  produced  by 
blood  extravasated  from  the  maternal  vessels  penetrating 
mto  the  substance  of  the  trophoblast  and  breaking  it  up 
mto  irregular  bands  and  processes  (Fig.  66,  A),  this  being 
possible  from  the  fact  that  even  at  this  early  stage  the 


Tin:  CHORION. 


»43 


ovum  is  completely  embedded  in  the  mucosa  of  the 
uterus.  In  later  stages  the  lacuiue  increase  in  size  and 
unite  to  form  an  extensive  blood  space  completely  sur- 
round". -  the  embryonic  vesicle.  Into  this  blood  space 
the  vessels  of  the  uterine  walls  open,  and  into  it  also  the 
irregular  processes  of  the  trophoblast  project,  forming 
what  arc  termed  the  chorionic  villi,  the  space  itself  being 
known  as  the  iutci villous  space  (I'ig.  66,  H). 


^^Tj/ 


Fig.  67.— Two  Viuu  from  the  Chorion  op  an  Embryo  ok  7  mm. 


These  villi  may  at  first  be  developed  over  the  whole 
surface  of  the  chorion  or  they  may  be  limited  to  a  broad 
band  situated  at  what  may  be  termed  the  equator  of  the 
ovum;  but  whichever  arrangement  occurs,  only  those  de- 
veloped from  that  portion  of  the  chorion  to  which  the 
belly-stalk  is  attached  undergo  further  elaboration  in 
later  stages,  the  rest  gradually  disappearing  or  remaining 
only  as  minute  rudiments.     It  is  customary,  consequently, 


I 

*    t. 

i  % 


» 


fr 


1 


\', 


144  THE    DEVELOPMENT    OF    THE    HUMAN    liODY. 


i.i 


Fig.  68.— Transverse  Sectio.vs  through  Chorionic  Vh,li  in  (A)  the 

Fifth  and  (B)  the  Seventh  Month  of  Development 

cf.  Canalized  fibrin;  fc  Langhans  cells;  s,  syncytiuni.-(^.  which  ts  more 

highly  magnified  than  /?.  /row  Szymonovicz;  B  from  Minot.) 


* 


THE   CHORION. 


H5 


to  Speak  of  that  portion  of  the  chorion  in  which  the  de- 
velopment of  the  villi  proceeds  as  the  chorion  frovdosutn,  to 
distinguish  it  from  the  remaining  portion,  which  is  termed 
the  chorion  Iceve. 

The  vilH  (Fig.  67)  at  first  are  irrpgrniylv  lobed  processes 
formed  by  a  solid  mass  of  troohoblast  cells  and  projecting 
fully  into  »^*>  ;n|fpn]]»"e  cpoo»  a  lobe  here  and  there 
extending  completely  across  the  space  (Fig.  76)  and  unit- 
ing with  the  maternal  tissues  to  form  roots  of  attachment. 
As  development  proceeds  the  lobes  become  much  more 
slender  and  branch  so  that  each  villus  assumes  a  dendritic 
form.  In  the  mean  time,  however,  processes  from  the 
chorionic  mp^nderm  ptow  out  into  each  villus,  extending 
out  even  mto  the  terminal  branches  and  forming  a  central 
core  in  which  blood-vessels  develop,  which  become  con- 
tinuous with  the  umbilical  arteries  and  veins.  When  this 
has  occurred,  the  ectoderm  differentiates  into  two  lavers. 
a  superficial  one  in  which  the  cell-boundarjes  disappear 
so  that  it  consists  of  a  continuous  layer  of  protoplasm  in 
wV.4r>Ti  niimf>rr^ii^  niiHpi  are  ppihedded  (Fig.  68,  A,  s)  and 
which  is  termed  the  syncytium,  and  aq^  inner  one,  consist- 
ing of  well-defined  cells  arranged  in  a  single  layer  and 
termed  the  Langhanscells  (Ic). 

It  may  be  stated  that  the  exact  significance  of  these  two 
layers  is  still  under  discussion,  some  authors  believing  the 
Langhans  cells  to  be  mesodermal,  while  others,  admitting 
that  they  are  ectodermal,  maintain  the  view  that  the  syn- 
cytium is  really  maternal  tissue.  The  view  here  presented  is 
most  in  accoid  with  the  more  recent  observations  (Minot, 
Peters). 

As  development  proceeds  the  villi,  which  are  at  first 
distributed  evenly  over  the  chorion  frondosum,  are 
separated  into  groups  termed  cotyledons  (Fig.  69),  by 
the  growth  into  the  intervillous  space  of  trabeculse  from 


SI 


146 


THE    DEVELOPMENT    OF    THE    HUMAN    UODV. 


the  walls  of  the  uterus,  the  villous  roots  of  attachment 
becoming  connected  with  these  septa  as  well  as  with 
the  general  uterine  wall.  The  ectoderm  of  the  villi  also 
undergoes  certain  changes  with  advancing  growth,  the 
layer  of  Langhans  cells  disappearing  except  in  small  areas 
scattered  irregularly  in  the  villi,  and  the  syncytium, 
though  persisting,  undergoes  local  thickenings  which  de- 
gener.',!-  more  or  less  extensively  into  fibrin-like  sub- 
stance^(Fig.  68,  B   -/). 


f  i  ■  r 

1:1 

ill 


r/i 


urn 


Fig.  69.---MATURE  Placenta  after  Separation  from  the  Uterus 
c,   Cotyledons;  ch,   chorion,   amnion,   and   decidua  vera;   urn,  umbilical 

cord.— (ATo/ZwaHw.) 

The  changes  which  occur  during  the  later  stages  of 
development  in  the  chorion  are  very  similar  to  those  de- 
scribed for  the  villi.  Thus,  the  mesoderm  thickens,  its 
outermost  layers  becoming  exceedingly  fibrillar  in  struc- 
ture, while  the  ectoderm  differentiates  into  two  layers, 
the  outer  of  which  is  syncytial  while  the  inner  is  cellular,' 
and  later  still,  as  in  the  villi,  the  syncytial  layer  degener- 
ates in  irregular  patches  into  a  peculiar  form  of  fibrin 


THE    DECIDU/F,. 


147 


which  is  traversed  by  flattened  anastomosing  spaces  and 
to  which  Minot  has  applied  the  name  canalized  fibrin 
(Fig.  70). 
The  Decidual. — In  connection  with  the  phenomenon  of 


mes 


JHf       ^^^ 


.•<«e*''-**fe.^ 


fb 


f^  ^''^..^v»^       ^P 


■^ 


% 


Fig.  70. — Section  through  the  Placental  Chorion  of  an  P'mbryo 

OF  Seven  Months. 
c,  Cell  layer;  f/>,   remnants  of  epithelium;  \h,  fibrin  layer;  mes,   meso- 
derm.— (A/tno<.) 


menstruation  periodic  alterations  occur  in  the  mucous 
membrane  of  the  uterus.  If  during  one  of  these  periods 
a  fertilized  ovum  reaches  the  uterus,  the  desquamation ;  of 


148 


THE    DEVELOPMENT   OF     THE    HUMAN    RODY. 


portions  of  the  epithelium  does  not  occur  nor  is  there  any 
appreciable  hemorrhage  into  the  cavity  of  the  uterus, 
the  uterine  mucosa  remains  in  what  is  practically  the  ante- 
menstrual  condition  until  the  conclusion  of  pregnancy, 
when,  after  the  birth  of  the  fetus,  a  considerable  portion  of 
its  thickness  is  expelled  from  the  uterus,  forming  what  is 


f 


!f 


h- 1 


t  ■ 


Fig.  71.— Diagram  showing  the  Rel.\tio.\s  ok  the  Fetal  Memtranes. 

Am,   Amnion;   Ch,   chorion;   M,    muscular  wall   of   uterus;  R,    decidua 

reflexa ;  5,  decidua  serotina ;  V,  decidua  vera ;  Y,  yolk-stalk. 

termed  the  deciduce.  In  other  words,  the  sloughing  of  the 
uterine  mucosa  which  concludes  the  process  of  menstrua- 
tion is  postponed  until  the  close  of  pregnancy,  and  then 
takes  place  simultaneously  over  the  whole  extent  of  the 
uterus.  Of  course,  the  changes  in  the  uterine  mucosa  are 
somewhat  more  extensive  during  pregnancy  than  during 


^ 


THE    DEClUU.i:. 


149 


menstruation,  but  there  is  an  undoubted  fundamental 
similarity  in  the  changes  during  the  two  processes. 

The  human  ovum  comes  into  direct  apposition  with 
only  a  small  portion  of  the  uterine  wall,  and  the  changes 
which  this  portion  of  the  wall  undergoes  differ  somewhat 
from  those  occurring  elsewhere.     Consequently  it  becomes 


—  d 


d 


Fi,;.  72.— Surface  View  of  Half  of  the  Decidua  Vera  at  the  End 

OF  the  Third  Week  of  Gestation. 
(/,    Mucous  membrane  of  the  Fallopian  tubes;  ds,  prolongation  of  the 

vera  toward  the  cervix  uteri;  pp,  papilla;;  rf,  marginal  furrow. — 

{Kollmann.) 


possible  to  divide  the  deciduae  into  (i)  a  portion  which  is 
not  in  direct  contact  with  the  ovum,  the  decidua  vera  (Fig. 
71,  y)  and  (2)  a  portion  which  is.  The  latter  portion  is 
again  capable  of  division.  The  ovum  becomes  com- 
pletely embedded  in  the  mucosa,  but,  as  has  been  pointed 


ISO 


TlfE    DEVELOPMENT    OF   THE    HUMAN    BODY. 


■! 

I  1 


I 


11 


out,  the  chorionic  vilH  reach  their  full  development  only 
over  that  pprtion  of  the  chorion  to  which  the  belly-stalk 
is  attached.  The  decidua  which  is  in  relation  to  this 
chorion  frondosum  undergoes  much  more  extensive  modi- 
fications than  that  in  relation  to  the  chorion  Iseve,  and  to 
it  the  name  of  decidua  serotina  (Fig. 
71, 5)  is  applied,  while  the  rest  of  the 
decidua  which  encloses  the  ovum  is 
termed  the  decidua  reflexa  (R). 

The  changes  which  give  rise  to  the 
decidua  vera  may  first  be  described 
and  those  occurring  in  the  others 
considered  in  succession. 

(a)  Decidua  vera. — On  opening  a 
uterus  during  the  fourth  or  fifth 
month  of  pregnancy,  when  the  de- 
cidua vera  is  at  the  height  of  its  de- 
velopment, the  surface  of  the  mucosa 
presents  a  corrugated  appearance  and 


W^^' 


Fig   73.— Diagrammatic  Sections  of  the  Uterine  Mucosa   A   in  the 
Non-pregnant  Uteris,  and  B,  at  the  Beginning  of  PREcfNANcv 

'''n,„=?S"'"i''°™P^''*"'"=   «'•    *^^    *^^^P^st    portions    of    the    glands-   m 
muscular  layer;  sp,  stratum  spongiosuni.-'(/^«„<i,a<  and  LldTn'n.)' 

is  traversed  by  irregular  and  rather  deep  grooves  (Fig. 
12).  This  appearance  ceases  at  the  internal  os,  the  mu- 
cous membrane  of  the  cervix  uteri  not  forming  a  dc- 


; 

A 


THE    DECIDU.F. 


151 


cidua,  and  the  dcciduae  of  the  two  surfaces  of  the  uterus 
are  separated  by  a  distinct  furrow  known  as  the  marginal 
groove. 

In  sections  the  mucosa  is  found  to  have  become  greatly 
thickened,  frequently  measuring  i  cm.  in  thickness,  and 
its  glands  have  undergone  very  considerable  modification. 
Normally  almost  straight  (Fig.  73,  A),  they  increase  in 
length,  not  only  keeping  pace  with  the  thickening  of  the 
mucosa,  bu*  surpassing  its  growth,  so  that  they  become 
very  much  contorted  and  are,  in  addition,  considerably 
dilated  (Fig.  73,  B).  Near  their  mouths  they  are  dilated, 
but  not  very  much  contorted,  while  lower  down  the  re- 
verse is  the  case,  and  it  is  possible  to  recognize  three  layers 
in  the  decidua,  ( i )  a  stratum  compactum  nearest  the  lumen 
of  the  uterus,  containing  the  straight  but  dilated  portions 
of  the  glands;  (2)  a  stratum  spongiosum,  so  called  from 
the  appearance  which  it  presents  in  sections  owing  to  the 
dilated  and  contorted  portions  of  the  glands  being  cut  in 
various  planes;  and  (3)  next  the  muscular  coat  of  the 
uterus  a  layer  containing  the  contorted  but  not  dilated 
extremities  of  the  glai\ds  is  found.  Only  in  the  last  layer 
docs  the  epithelium  of  the  glands  retain  its  normal  col- 
umnar form,  elsewhere  it  becomes  more  or  less  flattened 
and  shows  a  tendency  toward  degeneration. 

In  addition  to  these  changes,  the  epithelium  of  the 
mucosa  disappears  completely  during  the  first  month  of 
pregnancy,  and  the  tissue  between  the  glands  in  the  stra- 
tum compactum  becomes  packed  with  large,  often  multi- 
nucleated cells,  which  are  termed  the  decidual  cells. 

After  the  end  of  the  fifth  month  the  increasing  size  of 
the  embryo  and  its  membranes  exerts  a  certain  amount  of 
pressure  on  the  decidua,  and  it  begins  to  diminish  in  thick- 
ness. The  portions  of  the  glands  which  lie  in  the  stratum 
compactum   become   more   and    more   compressed   and 


^^? 


•5- 


THE    DEVELOPMENT    OF    THE    HUMAN    HODY. 


finally  disappear,  while  in  the  spon-iosum  the  spaces  be- 
come much  flattened  and  the  vascularity  of  the  whole 
decidua,  at  first  so  pronounced,  diminishes  greatly. 

(b)  Decidua  reflexa.—The  decidua  reflexa  receives  its 
name  from  the  fact  that  it  was  supposed  to  arise  as  a  fold 
of  the  mucous  membrane  of  the  uterus  and  to  be  reflected 


ScU. 


I 


H 


•  n 


I'f 


E.V. 


:^- ■■'■■:■  ,^^^1^:-*^:  •...•■'•■-••  "f-; 


pio.  74. — Section  of  an  Ovum  of  1  mm.     A  Section  of  the  Embryo 

Lies  in  the  Lower  Part  of  the  Cavity  of  the  Ovum. 
/),    Decidua;    E.U.,    uterine    epithelium;    Sch,    blood-clot    closing    the 

aperture  left  by  the  sinking  of  the  ovum  into  the  uterine  mucosa. 

— {From  Strahl,  ajtcr  Peters.) 

over  the  ovum  after  this  had  attached  itself.  Recent 
observations,  however,  throw  doubt  on  this  mode  of  origin. 
Thus,  the  ovum  described  by  Peters  (Fig.  74)  was  already 
almost  completely  enclosed  by  the  reflexa,  a  small  area 
at  one  pole  being  alone  exposed.     The  uterine  epithe- 


n 


rt 


THE    DECinUK. 


•53 


■E.U. 


Hum  around  the  margins  of  this  unenclosed  area  was 
exceedingly  thin  and  had  the  appearance  of  being 
stretched  by  the  growth  of  the  ovum.  Peters  interprets 
the  condition  found  in  this  very  early  stage  by  supposing 
that  when  the  ovum  reached  the  uterus  it  came  into  con- 
tact with  the  thickened  mucosa  at  a  point  where  the 
epithelium  had  been  thrown  off  and  at  once  proceeded 
to  embed  itself  in  the  substance  of  the  mucosa.  By  the 
time  it  reached  a  diameter  of  about  i  mm.  the  ovum 
was  almost  completely  embedded  and  the  mucosa  sur- 
rounding it  constituted  tlic  rcflexa.  According  to  this 
view,  which  seems  to  be  more  in  harmony  with  what  has 
been  observed,  there  is  no  formation  of  a  fold  and  no  re- 
flection over  the  surface  of  the  ovum,  but  the  reflexa  is  due 
to  the  ovum  becoming  embedded  in  the  substance  of  the 
nmcosa. 

As  development  proceeds  the  reflexa  eventually  com- 
pletely encloses  the  ovum,  the  point  of  union  of  the  edges 
of  the  aperture  through  which  the  ovum  sank  into  the 
mucosa  being  indicated  for  some  time  by  a  scar-like  mark. 

The  general  structure  of  the  reflexa  is  closely  similar  to 
that  described  for  the  vera,  but  as  the  ovum  increases  in 
size  it  becomes  thinner  and  thinner,  and  at  about  the  fifth 
month  has  come  into  contact  with  the  vera,  forming  a 
whitish  transparent  membrane  with  no  traces  of  either 
glands  or  blood-vessels,  and  very  possibly  it  eventually 
degenerates  and  completely  disappears  (Minot). 

(c)  Decidua  sejotina. — The  structure  of  the  serotina  up 
to  about  the  fifth  month  of  development  is  practically  the 
same  as  that  of  the  vera.  It  loses  its  epithelium  very 
early,  probably  before  the  attachment  of  the  ovum,  and 
the  glands  undergo  the  same  changes  as  in  the  vera,  so 
that  the  compactum  and  spongiosum  can  be  recognized. 
Beyond  the  fifth  month,  however,  there  is  a  great  differ- 
13 


154 


THE    I'l.ACENTA. 


155 


: 

I 
i 


ence  between  it  and  the  vera,  in  that,  being  concerned 
with  the  nutrition  of  the  embryo,  it  does  not  partake  of 
the  degeneration  noticeable  in  the  other  deciduae,  but 
persists  until  birth,  forming  a  part  of  the  structure  termed 

the  placenta. 

The  Placenta.— This  organ,  which  forms  the  connection 
between  ihejixnhx^si-and  the  paternal  tissues,  is  comp^'^^'^d 
of  two  parts,  separated  by  the  intervillous  space.  0.v« 
of  these  parts  is  of  embryonic  origin,  being  the  cjjflHpn 
frondosum,  while  the  other  belongs  to  the  maternal  tissues 
and  is  the  decidua  serotina.  Hence  the  terms  placenta 
jcetalis  and  placenta  utaina  frequently  applied  to  the  two 
parts.  The  fully  formed  placenta  is  a  more  or  less  dis- 
coidal  structure,  convex  on  the  surface  next  the  uterine 
muscularis  and  concave  on  that  turned  toward  the  em- 
bryo, the  umbilical  cord  being  continuous  with  it  near  the 
center  of  the  latter  surface.  It  averages  about  3.5  cm.  in 
thickness,  thinning  out  somewhat  toward  the  edges,  and 
has  a  diameter  of  15  to  20  cm.,  and  a  weight  varying  be- 
tween 500  and  1250  grams.  It  is  situated  on  one  of  the 
surfaces  of  the  uterus,  the  posterior  more  frequently  than 
the  anterior,  and  usually  much  nearer  the  fundus  than  the 
internal  os.  It  develops,  in  fact,  wherever  the  ovum 
happens  to  become  attached  to  the  uterine  walls,  and  oc- 
casionally this  attachment  is  not  accomplished  until  the 
ovum  has  descended  nearly  to  the  internal  os,  in  which 
case  the  placenta  may  completely  close  this  opening  and 
form  what  is  termed  a  placenta  prcevia. 

If  a  section  of  a  placenta  in  a  somewhat  advanced  stage 
of  development  be  made,  the  following  structures  may  be 

Fig.  75.     Sectiox  through  .\  ^^lacenta  of  SfatSn  Months'  Dsvelqp- 

MEN'f. 

Am,  Amnion;  cho,  chorion;  D,  lay6r  of  decidua  contfilning  the  uterine 
glands;  Mc,  muscular  coat  of  the  uterus;  Ve,  u  ilcrnal  blood-vessel; 
Vi,  stalk  of  a  villus;  vi,  villi  in  section.— (.l/mo<.) 


^_^ 


i 


156  THE    PI      t!0!MiNI     (il       rilK    IllMAN    HOl)V 

distinguished:  O  .  tlit  muht  'surface  there  will  be  a  deli- 
cate layer  reprcstntiriH  Ck  aaiajan  (Fig.  75,  Am),  and 
next  to  this  a  somewhat  ihi  ker  one  which  is  the  chorion 
(cho),  in  which  the  degenerative  changes  already  men- 
tioned may  be  observed.  vSucceedinj;  this  comes  a  much 
broader  area  composed  of  the  large  intervillous  blood 
space  in  which  lie  sections  of  the  villi  (vi)  cut  in  various 
directions.  Tlicii  follows  the  stratum  compactum  of  the 
serotina,  next  the  stratum  spongiosum,  next  the  outer- 
most layer  of  the  muco:,a  (/)"),  in  which  the  uterine  glands 
retain  their  epith.  liani,  and,  linally,  the  nmsculasis  uteri 
(Mc). 

These  various  structures  which  enter  into  the  composi- 
tion of  the  placenta  have,  for  the  most  part,  been  aire  ly 
described,  and  it  remains  here  only  to  say  a  few  words  con- 
cerning the  special  structure  of  the  serotinal  compact  am 
and  concerning  the  origin  of  tlie  intervillous  space  and  its 
relations  to  the  villi  and  the  maternal  vessels. 

From  the  surface  of  the  compactum  processes  arise, 
termed  septa,  which  project  into  tht  intervillous  spat  e, 
grouping  the  villi  into  cotyledons  and  giving  fixation  u> 
some  of  the  roots  of  attachment  01  the  villi  (Fig.  75). 
Throughout  the  greater  extent  of  t!  -r  nlaccnta  the  h<p?a 
do  not  reach  the  Mirface  of  the  chorion,  but ;  t  the  1  jeri-  >- 
ery,  throughout  a  ..arrow  zone,  tliey  do  come  into  c  i- 
tact  with  the  chorion  and  unite  lieneath  it  to  form  a  me  i- 
brane  which  has  be<  n  termed  the  lI  'sing  plafr.  Bern 
this  lies  the  peripheral  portion  of  the  inter  lilous  spa.  ^, 
which,  owing  to  the  arrangement  of  th  sep+a  m  this 
region,  appears  to  be  imj)erfectly  set  arater  rr-  «^'  rest 
of  tlie  space  and  forms  what  is  ternii  1  the  rii,i>  mus 

(Fig.  76). 

The  probable  origin  of  the  intervillou     s|»j*ce  by  the 
effusion  of  blood  from  the  maternal  vessi      in;*  the  sub- 


■«ra^ti^^lti«(»i<B^^Bri^L 


158 


THE    DEVELOPMENT    OF    THE    HUMAN    BCJDY. 


;i 


i    ! 


ii    I 

[  !     t 


Stance  of  the  trophoblast  and  the  subsequent  corrosion 
of  that  layer  has  already  been  described,  and  if  this  be  the 
true  method  of  its  development,  then  it  is  evident  that  the 
fetal  villi  are  in  direct  contact  with  the  maternal  blood 
contamed  in  the  space.     The  uterine  vessels  become  very 
much  enlarged  during  pregnancy  and  those  of  the  sero- 
tma  communicate  freely  with  the  intervillous  space,  so 
that  a  free  circulation  of  the  maternal  blood  through  the 
space  occurs.     The  villi  being  completely  immersed  in 
this  constantly  renewed  blood,  an  osmotic  interchange 
takes  place  between  the  maternal  blood  of  the  space  and 
the  fetal  blood  contained  in  the  vessels  of  the  villi,  the  ma- 
ternal blood  transmitting  the  nutritive  materials  neces- 
sary for  the  growth  of  the  embryo  and  receiving  the  waste 
products  of  the  fetal  metabolism.     And  it  is  only  in  this 
manner  that  the  nutrition  of  the  embryo  can  take  place 
since  nowhere  is  there  a  direct  communication  of  the  two 
vascular  systems. 

It  has  been  maintained  by  many  authors  that  the  inter- 
villous space  is  lined  throughout  by  a  layer  of  cells  continuous 
n  ,  u,      f^dothehum  of  the  maternal  vessels,   so  that  the 
fetal  blood  IS  separated  from  the  maternal,  not  onlv  by  the 
fetal  tissues  of  the  villi  but  also  by  a  layer  of  maternal  tissue 
(compare  what  is  said  m  the  small  print  on  page  145  concern- 
mg  the  homologies  of  the  ectodermal  layers  of  the  villi)       The 
presence  of  such  a  layer  is  certainly  what  might  be  expected 
since,  as  Oscar  Hertwig  has  well  expressed  it,  "the  employ- 
ment of  spaces  lying  outside  the  blood-courses  as  component 
parts  of  the  vascular  system  would  be  a  phenomenon  without 
analogy.       It  is  to  be  noted  that  the  arteries  and  veins  of  the 
serotina  do  not  communicate  by  means  of  capillaries,  but  by 
the  intervillous  space,  and  this  has  given  rise  to  the  theory 
that  the  space  is  to  be  regarded  as  an  enormously  cilareed 
capillary    m  which  case  it  should  be  lined  throughout  by 
maternal   endothelium.     Recent   observations   on   the   lower 
mammals,   especially  the  rodents   (rabbit,  guinea-nig    etc) 
seem  to  show,  however,  that  the  space  owes  its  origin  to  a 
true  effusion  of  maternal  blood,  and  the  evidence  furnished 


THE    PLACENTA. 


159 


hv  Peters  and  van  Hcukelom  from  the  study  of  its  formation 
S^verfearty  human  embryos  indicates  its  origin  m  the  human 
placenta  in  the  manner  described  above. 

The  Separation  of  the  Decidual  at  Birth.-At  parturi- 
tion, after  the  rupture  of  the  amnion  and  the  expulsion  of 
the  fetus,  there  still  remains  in  the  uterine  cavity  the 
decidu.e  and  the  amnion,  which  is  in  contact  but  not 
fused  v.xth  the  decidus.   A  continuance  of  the  uterine  con- 
trLtions.  producmg  what  are  termed  the  "after-pains, 
results  in  t'he  separation  of  the  placenta  from  the  uterine 
walls,  the  separation  taking  place  in  the  deep  layers  of  the 
spongiosum,  so  that  the  portion  of  the  mucosum  which 
contains  the  undegenerated  glands  remains  behind      As 
soon  as  the  placenta  has  separated,  the  separation  of  the 
decidua  vera  takes  place  gradually  though  rapidly     he 
line  of  separation  again  being  in  the  deeper  layers  of  the 
stratum  spongiosum,  and  the  whole  of  the  deciduae.  to- 
gether with  the  amnion,  is  expelled  from  the  uterus,  form- 
ing what  is  known  as  the  "  after-birth  " 

Hemorrhage  from  the  uterine  vessels  during  and  after 
the  separation  of  the  deciduae  is  prevented  by  the  contrac- 
tions of  the  uterine  walls,  assisted,  according  to  some  au- 
thors, by  a  preliminary  blocking  of  the  mouths  of  the 
uterine  vessels  by  certain  large  polynuclear  decidual  cells 
found  during  the  later  months  of  pregnancy  in  the  outer 
layers  of  the  serotina.     The  regeneration  of  the  uterine 
mucosa  after  parturition  has  its  starting-point  from  the 
epithelium  of  the  undegenerated  glands  which  persist,  this 
epithelium  rapidly  evolving  a  complete  mucosa  over  the 
entire  surface  of  the  uterus. 


i6o 


THE   DEVELOPMENT  OF    THE   HUMAN    BODY. 


LITERATURE. 
S.  VAN  HeukELOm:  "  Uebef  die  menschliche   Placentation,"  Archiv  fur 

Anat.  und  Physiol.,  Anat.  Abtk.,  1898. 
W.   His:  "Die  Umschliessung  der  menschlichen  Frucht   wahrend  der 

fruhesten  iieit  des  Schwangerschafts,"  Archiv  far  Anat.  und  Physiol., 

Anat.  Abih.,  1897. 

F.  Keibbl:  "Zur  Eriiwickelungsgeschichte  der  Placenta,"  Anat.  Anzeiger, 

IV,  1889. 
J.  Kollmann:  "Die  menschlichen  Eier  vun  6  mm.  Grosse,"  Archiv  jUr 

Anat.  und  Physiol.,  Anat.  Abth.,  1879. 
J.  Merttbns:  "Beitrage  zur  normalen  und  pathologischen  Anatomic  der 

menschlichen  Placenta,"  Zeitschrift  jur  Geburtshulfe  und  Gynaekol., 

XXX  and  xxxi,  1894. 
C.  S.  Minot:  "Uterus  and  Embryo,"  Journal  of  Morpkol.,  ii,  1889. 

G.  Paladino:  "Sur  la  gen^se  des  espaces  intervilleux  du  placenta  humain 

et  de  leur  premier  contenu,  comparativement  h.  la  m^me  partie  cliez 
quelques  mammifferes,"  Archives  Ital.  de  Biolog.,  xxxi  and  xxxii, 
1899. 

H.  Peters:  "Ueberdie  Einbettung  des  menschlichen  Eies  und  das  frflheste 
bisher  bekannte  menschliche  Plac^ntationsstadium,"  Leipzig  und 
Wien,  1899. 

C.  RuiE:  "Ueberdie  menschliche  Placentation,"  Zeitschrift  fur  Geburts- 
hulfe und  Gynaekol.,  xxxix,  1898. 

F.  Graf  SpeE:  "Ueber  die  menschliche  Eikammer  und  Decidua  reflexa," 
Verhandl.  des  Anat.  Gesellsch.,  xii,  1898. 

J.  C.  Webster:  "Human  Placentation,"  Chicago,  1901. 


i: 


%' 


PART  II. 
ORGANOGENY. 


i 


CHAFFER  VI. 

THE    DEVELOPMENT    OF    THE    INTEGUMEN- 
TARY SYSTEM. 

The  Development  of  the  Skin.-The  skin  is  composed  of 
two  embryologically  distinct  portions,  the  outer  epidermal 
layer  being  developed  from  the  ectoderm,  while  the  der- 
mal layer  is  mesenchymatous  in  its  origin  and  is  formed 
from  the  dermatomes  of  the  mesodermic  somites. 

The  ectoderm  covering  the  general  surface  of  the  body 
is,  in  the  earliest  stages  of  development,  a  single  layer  of 
cells,  but  at  the  end  of  the  first  month  it  is  composed  of 
two  layers,  an  outer  one,  the  epitrichium,  consisting  of 
slightly  flattened  cells,  and  a  lower  one  whose  cells  are 
larger  and  which  will  give  rise  to  the  epidermis  (Fig.  77, 
A).  During  the  second  month  the  differences  between 
the  two  layers  become  more  pronounced,  the  epitrichial 
cells  assuming  a  characteristic  domed  form  and  becominp^ 
vesicular  in  structure  (Fig.  77.  B).  These  cells  persist 
until  about  the  sixth  month  of  development,  but  after  thai 
they  are  cast  off,  and,  becoming  mixed  with  the  secretion 
of  sebaceous  glands  which  have  appeared  by  this  time, 
form  a  constituent  of  the  vernix  caseosa. 

In  the  mean  time  changes  have  been  taking  place  in  the 
epidermal  layer  which  result  in  its  becoming  several  layers 
14  161 


i6: 


THE    nEVELOrMENT    OF     THE    HUMAN    BODV. 


thick  (Fig.  77,  B),  the  innermost  layer  being  composed  of 
cells  rich  in  protoplasm  while  those  of  the  outer  layers  are 
irregular  in  shape  and  have  clearer  contents.  As  develop- 
ment proceeds  the  number  of  layers  increases  and  the 
superficial  ones,  undergoing  a  horny  degeneration,  give 
rise  to  the  stratum  corneum,  while  the  deeper  ones  be- 
come the  stratum  Malpighii.  At  about  the  fourth  month 
ridges  develop  on  the  under  surface  of  the  epidermis,  pro- 


f  i 


t^ 


I'lci.  //'.     .1,  Section  ok  Skin  i  kom  the  Dorsum  of  Finger  of  an  Km- 
BKVo  of  4.5  CM.;  li,  from  the  Plantar  Surface  of  the  Foot  of 

AN    IvMBRVO   OF    10.2    CM 

1 1,   Hpilrioliiuiii ;  p/',  cpidtTinis 


jecting  downward  into  the  dermis  H^g.  8j),  and  later 
secondary  ridges  appear  in  the  intervals  between  the 
primary  ones,  wliile  <m  the  palms  and  soles  ridges  appear 
upon  the  outer  surfa<'  of  the  epidermis,  corresponding  in 
[MVvition  to  tht  phinarv  ridges  of  the  U'lder  surface. 

The  meseslch}^ne  derived  from  the  dermatomes  early 
loses  all  tr».f  s  of  its  <*iginal  segmental  arrangement  and 


I  THE   SKIN. 

forms  a  continuous  layer  underlying 
the    epidermis.       It    becomes    con- 
verted principally  into  fibrous  con- 
nective  tissue,  the    outer   layers    of 
which  are  relatively  compact,  while 
the  deeper  ones  are  looser,  forming 
the     subcutaneous     areolar     tissue. 
Some   of    Miese    mesenchymal   cells, 
however,     become     converted     into 
non-striated  muscle-fibers,  which  for 
the  most  part  are  few  in  number  and 
associated   with    the    hair    follicles, 
though   in   certain   regions,  such    as 
the  skin  of  the  scrotum,  they  are  very 
numerous  and  form  a  distinct  layer 
known    as    the   dartos.     Some   cells 
also  arrange  themselves  in  groups  and 
undergo  a  fatty  degeneration,  well- 
defined  masses  of  adipose  tissue  em- 
bedded in   the  lower  layers  of  the 
dermis  being  thus  formed  at  about 
the  sixth  month. 

Since  the  dermal  mesenchyme  is  pri- 
marih-  segmental  in  character,  it  might 
be  expected  that  indications  of  this  origi- 
nal condition  might  he  shown  by  the  dis- 
tribntion  (-;  the  cutaneous  nerves  in  the 
adult,  even  though  the  boundaries  of  each 
dermatome  had  become  indistinct.  A 
studv  of  the  cutaneous  nerve  supply  in 
the  adult  realizes  to  a  very  considerable 
extent  this  expectation,  the  areas  sup- 
plied   by   the    various    nerves    forming 


Fin.  78.     DiACRAM  showint,   thb  CiTANEors 

DlSTKIBITION     Ol-     THK    vSl'lNAt   NEKVES. — 


163 


'/•^  ^J 


^Ts, 


TV 


Ts 


Ts 


M 


Ta 


Tio 


Tl2^ 


^Tit 


Si 


Lt 


IJ 


lii:::!^ 


v7^f,""Zii.f^mi 


I 


164  THE    DEVEI-OPMENT   OF    THE    HUMAN    BODY. 

more  or  less  distinct  zones,  and  being  therefore  segmented 
(Fie  78)  But  a  considerable  commingling  of  adjacent 
dermatomes  has  also  occurred.  Thus,  while  the  distribution 
of  the  cutaneous  branches  of  the  fourth  thoracic  nerve,  as 
determined  experimentally  in  the  monkey  {Macacus)  is  dis- 
tinctly zonal  or  segmental,  the  nipple  lying  practically  i>i  the 
middle  line  of  the  zone,  the  upper  half  of  its  area  is  also  supplied 
(,r  overlapped  bv  fibers  of  the  third  nerve  and  the  ower  half 
bv  fibers  of  the  "fifth  (Fig.  79).  «>  that  any  area  of  skm  in  the 
zone  is  innervated  bv  fibers  coming  from  at  least  two  segmental 
nerves  (vSherringtcm).  And.  furthermore,  the  distribution  of 
each  nerve  crosses  the  mid-ventral  line  of  the  body,  forming  a 
more  or  less  extensive  crossed  overlap. 

\nd  not  onlv  is  there  a  confusion  of   adjacent  dermatomes 
but   a  dermatinne    may   shift    its   position  relatively   to  the 


I  % 


Mi 


Fir.   79    -  l)i.\c.R.\M  showim;  thk  Overlap  of  the  ///,  IV,  and  V  Inter- 
cosT.VL  .Nerves  ok  .\  Monkey.     (Shcnini^ton.) 

deeper  structure  supplied  by  the  same  nerve,  so  that  the 
skin  over  a  certain  muscle  is  not  necessarily  supplied  by 
fibers  from  the  nerve  which  supplies  the  muscle.  Thus  in  the 
lower  half  of  the  abdomen,  the  skin  at  any  point  will  be  sup- 
plied bv  fibers  from  higher  nerves  than  those  supplying  the 
underlving  muscles  wSherrington).  and  the  skin  of  the  limbs 
mav  receive  twigs  from  nerves  which  are  not  represented  at 
all  in  the  muscle  supply  (second  and  third  thoracic  and  third 
sacral). 

The  Development  of  the  Nails.— The  earliest  indications 
of  the  development  of  the  nails  have  been  described  by 
Zander  in  embryos  of  about  nine  weeks  as  slight  thicken- 


THE   NAILS. 


165 


ines  of  the  epidermis  of  the  tips  of  the  digits,  these  thick- 
enings being  separated  from  the  neighboring  tissue  by  a 
faint  groove.  Later  the  nail  areas  migrate  to  the  dorsal 
surfaces  of  the  terminal  phalanges  (Fig.  80)  and  the 
grooves  surrounding  the  areas  deepen,  especially  at  their 
proximal  edges,  where  they  form  the  nai7-/oW.(n/),  while 
distallv  thickenings  of  the  epidermis  occur  to  form  what 
have  been  termed  sole-plates  (sp),  structures  qmte  rudi- 


-'iii^ri,-.'?/' 


■J,^'*' 


-^-*' 


Fir.    80  — T.oNoiTtMiNAu  Section  throtch   the  Terminal  Joint  of 

THE  Index-finger  of  an  Embryo  of  4.5  cm. 
,•     Kpidennis;  cp,  epilrichium;  h/,  nail  fold;  Ph,  terminal  phalanx;  sp, 

sole  plate. 

mentary  in  man,  but  largely  developed  in  the  lower  ani- 
mals, in  which  they  form  a  considerable  portion  of  the 

claws. 

The  actual  nail  substance  does  not  form,  however,  until 
the  embryo  has  reached  a  length  of  about  1 7  cm.  By  this 
time  the  epidermis  has  become  several  layers  thick  and  its 
outer  layers,  over  the  nail  areas  as  well  as  elsewhere,  have 


1 66 


THF.    nEVF.r.OPMENT  OF    THF.    HL'MAN    BODY. 


I    !' 


become  transformed  into  the  stratum  corneum  (Fig.  8i, 
sc),  and  it  is  in  the  deeper  layers  of  this  that  keratin  gran- 
ules develop  in  cells  which  degen- 
erate to  give  rise  to  the  nail  sub- 
stance (n).  At  its  first  formation, 
accordingly,  the  nail  is  covered  by 
the  outer  layers  of  the  stratum 
corneum  as  well  as  by  the  epitri- 
chium,  the  two  together  forming 
what  has  been  termed  the  epony- 
cliiuni  (Fig.  8i,  ep).  The  epitri- 
chium  soon  disappears,  however, 
leaving  only  the  outer  layers  of  the 
stratum  corneum  as  a  covering,  and 
this  also  later  disappears  with  the 
exception  of  a  narrow  band  sur- 
rounding the  base  of  the  nail  which 
persists  as  the  perionyx. 

The  formation  of  the  nail  begins 
^^  P  V'l  ij  ^"  ^^^^  more  proximal  portion  of 

the  nail  area  and  its  further 
growth  is  by  the  addition  of  new 
keratinized  cells  to  its  proximal 
L'(\^Q  and  lower  surface,  these  cells 
Ik'  A' ■'  J  being  formed  only  in  the  proximal 

part  of  the  nail  bed  in  a  region 
marked  by  its  whitish  color  and 
termed  the  lunula. 


% 


f-:--4 


/life 


FlO.     81.  —  I^ONf.lTlDINAU 

Section  thkoigu  thk 
Nail  Area  in  an  Km- 

BK\(I  OF    t"   (.M. 

f/>,  ICponycliium  ;  ti,  nail 
siil)Stance;  h/>,  nail  bed; 
sc,  stratum  corneiini; 
sp,  sole  plate. — {Oku- 
mtira  ) 


The  first  appearance  of  the  nail  areas 
at  the  tips  of  the  digits  as  described 
by  Zander  has  not  yet  been  con- 
firmed by  later  observers,  but  the  mi- 
gration of  the  areas  to  the  dorsal  sur- 
face necessitated  by  such  a  location 
of  the  primary  differentiation  affords 


THE    HAIR. 


167 


an  exDlaiiation  of  the  otherwise  anomalous  cutaneous  nerve- 
supply  of  the  nail  areas  in  the  adult,  this  be.ng  from  the 
palmar  (plantar)  nerves. 

The  Development  of  the  Hairs.-  The  hairs  bcRin  to 
develop  at  about  the  third  month  and  continue  to  be 
formed  during  the  remaining  portions  of  fetal  life.     They 


m. 


Fio.  82.     The   DfiVELorMKNT  of  a   Hair. 
c    Cvlindrical  cells  of  :,ir:Uuin  mucosuin;  /«/,   wall  of   hair  follicle;    w, 
'      "mesoderm :  wm,  stratum  tmicosum  of  epidermis;   />,  hair  papilla;   r, 
root  of  hair;  s,  sebaceous  gland.— (R'o//>n<JHn.) 

arise  as  solid  cylindrical  downgrowths.  projecting  ob- 
liquely into  the  subjacent  dermis  from  the  lower  surface 
of  the  epidermis.  As  these  downgrowths  continue  to 
elongate,  they  assume  a  somewhat  club-shaped  form  (Fig. 
82),  and  later  the  extremity  of  each  club  moulds  itself 


1 68 


THE    PEVELOPMENT   OF     THE    HUMAN    nODV, 


i  I 


over  ine  summit  of  a  small  papilla  which  develops  from 
the  dermis  (Fig.  82).  livcn  before  the  dermal  papilla 
has  made  its  appearance,  however,  a  differentiation  of 
the  cells  of  the  downgrowth  becomes  evident,  the  central 
cells  becoming  at  first  spindle-shaped  and  then  undergo- 
ing a  keratinization  to  form  the  hair  shaft,  while  the  more 
peripheral  ones  assume  a  cuboidal  form  and  constitute 
the  lining  of  the  hair  follicle.  The  further  growth  of  the 
hair  takes  place  by  the  addition  to  its  basal  portion  of 
new  keratinized  cells,  probably  produced  by  the  multipli- 
cation of  the  epidermal  cells  which  envelop  the  papilla. 

From  the  cells  which  form  the  lining  of  each  follicle  an 
outgrowth  takes  place  into  the  surrounding  dermis  to 
form  a  sebaceous  gland,  which  is  at  first  solid  and  club- 
shaped,  though  later  it  becomes  lobed.  The  central  cells 
of  the  outgrowth  separate  from  the  peripheral  and  from 
one  another,  and,  their  protoplasm  undergoing  a  fatty 
degeneration,  they  finally  pass  out  into  the  space  between 
the  follicle  walls  and  the  hair  and  so  reach  the  surface,  the 
peripheral  cells  later  giving  rise  by  division  to  new  genera- 
tions of  central  cells.  During  fetal  life  the  fatty  material 
thus  poured  Dut  upon  the  surface  of  the  body  becomes 
mingled  with  tlie  cast-off  '-pitrichial  cells  and  constitutes 
the  white  oleaginous  subst.mce,  the  vernix  caseosa,  which 
cover  the  surface  of  the  new-born  child.  The  muscles, 
arrectores  pilorum,  connected  with  the  hair  follicles  arise 
from  the  mesenchyme  cells  of  the  surrounding  dermis. 

The  first  growth  of  hairs  forms  a  dense  covering  over  the 
entire  surface  of  the  fetus,  the  hairs  which  compose  it 
being  exceedingly  fine  and  silky  and  constituting  what  is 
termed  the  lanugo.  This  growth  is  cast  off  soon  after 
birth,  except  over  the  face,  where  it  is  hardly  noticeable 
on  account  of  its  extreme  fineness  and  lack  of  coloration. 
The  coarser  hairs  which  replace  it  in  certain  regions  of  the 


Till     SUnORIPAROUS   GLANDS. 


169 


il 


body  probably  arise  from  new  follicles,  since  the  formation 
of  follicles  takes  place  throUKhout  the  later  periods  of  feta 
life  and  possil,lv  after  birth.  Hut  even  these  later  formed 
hairs  do  not  .<lividually  persist  for  any  great  length  of 
time  but  are  continually  being  shed,  new  or  secondary 
hairs  normally  deyeloping  in  their  places.  The  shedding 
of  a  hair  is  preceded  by  a  cessation  of  the  proliferation  ot 

the  cells  coyerim;  the  dermal  papilla  and  by  a  shrinkage  of 

the   papilla    whereby    it 

becomes   detached   from 

the  hair,  and  the  replac- 
ing hair  arises  from  a  pa 

pilla  which   is   probably 

budded  off  from  the  older 

one  before  its  degeiiera 

tion  and  carries  with  it  ;i 

cap  of  epidermal  cells. 

It  is  uncertain  whether 
the  cases  of  excessive  de- 
velnpment  of  hair  over  the 
face  and  upper  part  of  the 
body  which  occasionally 
occur  are  due  to  an  ex- 
cessive development  of  the 

later  hair  follicles  (hvpertrichosis)  or  to  a  persistence  and  con- 
tinued growth  of  the  lanugo. 

The   Development  of  the  Sudoriparous  Glands.— The 

sudoriparous  glands  arise  during  the  fifth  month  as  solid 
cylindrical  outgrowths  from  the  primary  ridges  of  the  epi- 
dermis (Fig.  83),  and  at  first  project  verticallv  downward 
into  the  subjacent  dermis.  Later,  however,  the  lower 
end  of  eacli  downgrowth  is  thrown  into  coils,  and  at  the 
same  time  a  lumen  appears  in  the  center.  Since,  how- 
ever, the  cylinders  are  formed  from  the  deeper  layers  of 
the  epidermis,  their  lumina  do  not  at  first  open  upon  the 


I'k;.  8,V-  Lower  Sikhace  01  a  De- 
tached I'ORTION  OH  lU'lDEKMlSFRPM 
THE   DORSl  M  OF  THE   HaNI>. 

/)    Hair   foUiclf;    t,  sudoriparous  Rimd. 

--(KItiscliki'.) 


I70 


TDK    HKVKI.OPMF.NT    OF   TIIK    HUMAN    ItOI>V. 


,4 

i 


surface,  but  gradually  approach  it  as  the  cells  of  the  deeper 
layers  of  the  epidennis  replace  those  which  ai  continually 
beinjf  cast  off  from  the  surface  of  tiie  stratum  corneum. 
The  final  opening?  to  the  surface  occurs  durinij  the  seventh 
month  of  development. 

The  Development  of  the  Mammary  (ilands.--In  the 
majority  of  tl.o  lower  mannnals  a  number  of  manunary 
glands  occur,  arranged  in  two  longitudinal  rows,  and  it  has 
been  observed  that  in  the  pig  the  first  indication  of  their 

developnu-nt  is  seen  in 
a  thickening  of  the  epi- 
dermis along  a  line 
situated  at  the  junction 
of  the  abdominal  walls 
with  the  membrana  re- 
uniens  fSchulze).  This 
thickening  subsequent- 
ly becomes  a  pro- 
nounced ridge,  the 
milk  ridge,  from  which 
at  certain  points  the 
mammary  glands  de- 
velop, the  ridge  disap- 
pearing in  the  inter- 
vals. In  a  human 
embryo  4  mm.  in  length  an  epidermal  thickening  has  been 
observed  which  extended  from  just  below  the  axilla  to  the 
inguinal  region  (Fig.  84)  and  was  apparently  equivalent 
to  the  milk  line  of  the  pig,  and  in  embryos  of  14  or  15  mm. 
the  upper  end  of  the  line  had  become  a  pronounced  ridge, 
while  more  posteriorly  the  thickening  had  disappeared. 

The  further  history  of  the  ridge  has  not,  however,  been 
yet  traced  in  human  embryos,  and  the  next  stage  of  the 
development  of  the  glands  which  has  been  observed  is  one 


ffff 


Fir,.  84. 


-MiUK  RiiK.E  (mr)  in  a  HtM.w 

H.MBKVO.-     (A.'(j//(HV.) 


TIIF.    MAMIARY    i^LANPS. 


(Upression  of  ! he  surface  of  the  skin      1  )nrinR  tlie  lift h  and 


'try  ••  fjU***? 


ti^- 


.1? 


^^rt5SJ::?*.yi 


Fio   85  -  Sections  through  the  KrinERMAU  Thickemn<.s  which  Re- 
present THE  Mammary  Guand  in  Ivmbryos  (A)  ov  6  cm.  anh  (/>) 

OF  10.2  CM. 

sixth  months  the  lobes  elongate  into  solid  cylindrical  col- 
umns of  cells  (Fig.  86)  resembling  not  a  little  the  cylinders 
which  become  converted  into  sudoriparous  glands,  and 
each  column  becomes  slightly  enlarged  at  its  lower  end, 
from  which  outgrowths  begin  to  develop  to  form  tlie  acini. 


172  THE    DEVELOPMENT   OF    THE   HUMAN    BODY. 

A  lumen  first  appears  in  the  lower  ends  of  the  columns  and 
is  formed  by  the  separation  and  breaking  down  of  the 
central  cells,  the  peripheral  cells  persisting  as  the  lining 
of  the  acini  and  ducts. 

The  elevation  of  the  gland  area  above  the  surface  to 
form  the  nipple  appears  to  occur  at  different  periods  in 
diflFerent  embryos  and  frequently  does  not  take  place  until 
after  birth.  In  the  region  around  the  nipple  sudoriparous 
and  sebaceous  glands  develop,  the  latter  also  occurring 
within  the  nipple  area  and  frequently  opening  into  the 
extremities  of  the  lacteal  ducts,  .n  the  areola,  as  the 
area   surrounding   the   nipple   is   termed,   other  glands. 


I'Ki.  86. -Section  throi(;h  the  Mammary  Gland  of  ,\n  Kmbryoof  25  cm. 
1,  Stroma  of  tlie  gland.— (/Vom  \agel,  after  liascli.) 

known  as  Montgomery s  glands,  also  appear,  their  develop- 
ment resembling  that  of  the  mammary  gland  so  closely  as 
to  render  it  probable  that  they  are  really  rudimentary 
mammary  glands. 

The  further  development  of  the  glands,  consisting  of  an 
increase  in  the  length  of  the  ducts  and  the  development 
from  them  of  additional  acin. ,  continues  slowly  up  to  the 
time  of  puberty  in  both  sexes,  but  at  that  period  further 
growth  ceases  in  the  male,  while  in  females  it  continues 
for  a  time  .md  the  subjacent  dermal  tissues,  especially  tlie 
adipose  tissue,  undergo  a  rapid  development. 


LITERATURE. 


173 


t  The  occurrence  of  a  milk  ridge  has  not  yet  bec-ii  observed 
in  a  sufficient  number  of  embryos  to  determine  whether  it  is 
a  normal  development  or  is  associated  with  the  formation 
of  supernumerary^  glands  {polymastia)  This  is  by  no  means 
an  infrequent  anomalv;  it  has  been  observed  in  19  per  cent, 
of  over  100,000  soldiers  of  the  German  army  who  were  ex- 
amined, and  occurs  in  47  per  cent,  of  mdividuals  in  certain 
regions  of  Germany.  The  extent  to  which  the  anomaly  is 
dt^veloped  varies  from  the  occurrence  ol  well-developed  acces- 
sory glands  to  that  of  rudimentary  accessory  nipp  es  {hyt>er- 
thclia)  these  latter  sometimes  occurring  in  the  areolar  area  ot 
a  normal  g'and  and  being  possibly  due  in  such  cases  to  an 
hvpertrophv  of  one  or  more  of  Montgomer>''s  glands. 

Although  the  mammarv  glands  are  typically  functional 
onlv  in  females  in  the  period  immediately  succeeding  preg- 
nancv  cases  are  not  unknown  in  which  the  glands  have  been 
well  developed  and  functional  in  males  {gymccomasha).  1-ur- 
thermore,  a  functional  activity  of  the  glands  normally  occurs 
immediately  after  birth,  infants  of  both  sexes  yielding  a  few 
drops  of  a  milky  fluid,  the  so-called  witch-milk  (Hexenmilch), 
when  the  glands  are  subjected  to  pressure. 

LITERATURE. 

R.   hoNNKT;  "Die  Mamn-.arorRane  iiti  Lichte  dcr  Ontogenie  und   Pliylu- 

genie,"  Erf^ihnisse  dcr  Anat.  iiiid  Enluickiluni^s^iscli.,  11,   184,^. 
J.  T.  Bowen:  "The  Epilrichial  Layer  of  the  Human  Kpidermis,"  Atml. 

Amii^cr,  iv,  1889. 
C.   HiRCKHAKr:    "I'eber   enibryonale    Hyi)ermastie    und    Hy])erthehe," 

Anat.  Hifte,  viii,   '897. 
H.  Head:  "On  Disturbances  of  Sensation  with  Special  Reference  to  the 

Pain  of  Visceral   Disease,"  l^rani,   xvi,    18')2;  xvii,    1894;  and   xix, 

1896. 
K.  Kaluus.  "Kin  Kail  v.m  Milchleiste  bei  eineiii  menschlichen  linibryo," 

Anat.  llcjte,  viii,   1897. 
T.   Okami-ra:   "Ueber   die    Kntvvicklung    des    Nagels   beiin    Menschen," 

Archiv  fiir  llcrmntol.   und  Sypliilnl  .   xxv,    1900. 
H.  .Schmidt:  "I'eber  noriuale  HypertheHe  niensclihcher  lunbry.men  und 

uber  die   erste   Anlage   der   menschlichen    Milchdriisen   iiberhanpt,  ' 

Morphol.-Arhcitcn,  xvil,   1897. 
C.    S.    Sherrington:  "Kxi)eriments  in    Examination   of   the    Ferii)heral 

Distribution  of  tiie   Kibres  of  the   Posterior    Root.,  of  some  Spinal 

Nerves,"    I'Uilosoph.    Tran^.    /-.'.'.Vd/   Soc,   ci.xxxiv,    189.^,   and   cxc, 

1898. 
H.  Stkaiil;    -Die  crslc  lintwicklun^;  der  Mammarorj^ane  beini  Menschen," 

Wrhandl.  Anat.  Ge.scllsch.,  xii,  1898. 


CHAPTER  VII. 


r>  Bf 


C!  ! 


fi>! 


11 


I 


THE    DEVELOPMENT     OF    THE    CONNECTIVE 
TISSUES  AND  SKELETON. 

It  has  been  seen  that  the  cells  of  a  very  considerable 
portion  of  the  somatic  and  splanchnic  mesoderm,  as  well 
as  of  parts  of  the  mcsodermic  somites,  become  converted 
into  mesenchyme.     A  very  considerable  portion  of  this 
becomes  converted  into  what  are  termed  connective  or 
supporting  tissues,  characterized  by  consisting  of  a  non- 
cellular  matrix  in  which  more  or  less  scattered  cells  are 
embedded.     These  tissues  enter  to  a  greater  or  less  extent 
into  the  formation  of  all  the  organs  of  the  body,  with  the 
exception  of  those  forming  the  central  nervous  system, 
and  constitute  a  network  which  holds  together  and  sup- 
ports the  elements  of  which  the  organs  are  composed; 
in  addition,  they  take  the  form  of  definite  membranes 
(serous  membranes,  fasciae),  cords  (tendons,  ligaments), 
or  solid  masses  (cartilage ),  or  form  looser  masses  or  layers 
of  a  somewhat  spongy  texture  (areolar  tissue).     The  in- 
termediate substance  is  somewhat  varied  in  character, 
being  composed  sometimes  of  white,  non-branching  non- 
elastic   libers,   sometimes   of   yellow,  branching,   elastic 
fibers;  of  white,  !)ranching,  but    inelastic    fibers   which 
form    a    reticulum,  or   of   a    soft    gelatinous    substance 
containing  considerable  (juantities  of  mucin,  as  in  the  tis- 
sue which  constitutes  the  Wliartonian  jelly  of  the  umbili- 
cal cord.     Again,  in  cartilage  the  matrix  is  compact  and 
homogeneous,  or,   in  other  cases,  more  or  less  fibrous, 
passing  over  into  ordinary  librous  tissue,  and,  final!}',  in 

»74 


.^ 


THE   CONNECTIVE  TISSUES. 


175 


bone  the  organic  matrix  is  largely  impregnated  with  salts 
of  lime. 

Two  views  exist  as  to  the  mode  of  formation  of  the 
matrix,  some  authors  maintaining  that  in  the  fibrous  tis- 
sues it  is  produced  by  the  actual  transformation  of  the 
mesenchyme  cells  into  fibers,  while  others  claim  that  it 
is  manufactured  by  the  cells  but  docs  not  directly  repre- 
sent the  cells  themselves.  Fibrils  and  material  out  of  which 
fibrils  could  be  formed  have  undoubtedly  been  observed 
in  connective-tissue  cells,  but  wliether  or  not  these  are 


G<? 


'V? 


^^vi^  Ifylff  L;, 


I'iG.  87.— Portion  of  the  Center  of  Ossification  ov  the  Parietal 
Hone  of  a  Himan  ICmbryo. 


later  passed  to  the  exterior  of  the  cell  to  form  a  conncetive- 
tissue  fiber  is  not  yet  certain,  and  on  this  hangs  mainly 
the  difference  between  tlie  theories.  Recently  it  has  been 
held  (Mall)  that  the  mesenchyme  of  the  embryo  is  really 
a  syncytium  in  and  from  the  protoplasm  ol  which  the 
matrix  forms;  if  this  be  correct,  the  distinction  which  the 
older  views  make  between  the  intercellular  and  intra- 
cellular origin  of  the  matrix  becomes  of  little  importance. 

Bone  differs  from  the  other  varieties  of  connective  tis- 
sue in  that  it  is  never  a  primary  formation,  but  is  always 


^J 


I 


,;6  THK    OKVF.LOPMENT    OF     THE    HUMAN    BODY. 

developed  either  in  fibrous  tissue  or  cartilage ;  and  accord- 
ing as  it  is  associated  with  the  one  or  the  other  it  i. 
spoken  of  as   tni^mjminebo^^  in  the 

development  of^^^^^^^^TSF^HTbone  some^^mre  connective- 
tissue  cells,  which  in  consequence  become  kno-n  as 
o9/roWa.s/.s.  deposit  lime  salts  in  the  matrix  in  the  form  of 

bony  spicules   which   in- 
crease  in  size   and    soon 
unite  to  form  a  network 
(Fig.  87).     The  trabecu- 
lae  of   th<'  network  con- 
tinue to  thicken,  while,  at 
the  same  time,  the  forma- 
tion of  spicules  extends 
further  out  into  the  con- 
nective-tissue membrane, 
radiating  in  all  directions 
from  the  region  in  which 
it  first  developed.     Later 
tiie      connective      tissue 
which    lies    upon    either 
surface    of   the    reticular 
plate' of   bone  thus  pro- 
duced condenses  to  form 
a    stout    membrane,  the 
periosteum,    between 
which    and    the    osseous 
plate  osteoblasts  arrange 
thetnselves  in  a  more  or  less  definite  layer  and  deposit 
po     the    surface   of    the   plate   a   lamella   of   compac 
Zl    A  membrane  V,one,  such  as  one  of  the  flat  bones  of 
he  skull  thus  comes  to  be  compose.l  of  two  plates  of  com- 
et   bone,  the   inner   .nd   outer   tables,  enclosing  and 
llnited  to  a  middle  plate  of  spongv  bone   which  const, 
tutcs  the  diploe 


l-i,,    88      l,(.N..iTri>iN.\L  Section  «Ji- 

I'llAUANX   Ol-     A    FlN'.lvK   Ol-    .SN     I'-M 

HKVo  OF   U   Month- 
.,    Cartilage    irahccula- ;    /-,   fen-sU-al 
t,..nc;  ^^  luTiostcut.i ;  y.  MssilKati..n 
center.     {Szymonouicz.) 


*DdA.-* 


TUK    DKVELOPMENT    OK    BONE. 


177 


t 


cc 


po 


m 


P^ 


With  bones  formed  from  cartilaj,a-  the  process  is  some- 
what different.     In  the  center  of  the  cartilage  the  inter- 
cellular matrix  becomes  increased  so  tlu.^  the  cells  appear 
to  be  more  scattered  and  a  calcareous  deposit  forms  in  it. 
All  around  this  region  of  calcihcation  the  cells  arrange 
themselves  in   rows  (Fig.  88)  and  the  process  of  calci- 
fication extends   into  the 
trabecular  of  matrix  wliich 
separate  these  rows.  While 
these  processes  have  beeti 
taking   place    the    mesen- 
chyme    surrounding     the 
cartilage  has  become  con- 
verted into   a  periosteun> 
ipo),    similar    to    that    of 
membrane    bone,  and    its 
osteoblasts  deposit  a  layer 
of  bone  ip)  upon  the  sur- 
face of  the  cartilage.     Tlie 
cartilage  cells   now  disap- 
pear from  the  intervals  be- 
tween   the     trabecular    of 
calcified     matrix,     which 
form  a   fine  network  into 
which    masses    of    mesen- 
chyme (Fig.  89,  pi),  con- 
taining blood-vessels  and  osteoblasts,  here  and  there  pen- 
etrate from  the  periosteum,  after  having  broken  through 
the  layer  of  periosteal  bone.  These  masses  absorb  portions 
of  the  fine  calcified  network  and  so  transform  it  into  a 
coarse  network,  the  meshes  of  which  they  occupy  to  form 
the  bone  marrow  (>«),  and  the  osteoblasts  which  they  con- 
tain arrange  themselves  on  the  surface  of  the  persisting 
trabecular  and  deposit  layers  of  bone  upon  their  surfaces. 


m^^^i^t. 


Fi( 


ThK  OSSIIMCATIO.N  CK.N'TER 

.  88  MoKiv  Hu.HUV   M.\(;.Ni- 


89.- 

OF    Fit 
KIEI). 

Ossifying  traheculas  cc,  cavity  of 
cartilaj;t'  network;  m,  marrow 
cells;  /),  periosteal  hone;  />/,  ir- 
rii])tion  of  ])eriosteal  tissue;  /'n, 
])eriostciiiii.      {S3ymontniic:.) 


178 


IllE    OKVELOl'MENT    OF    THE    HUMAN    BODY. 


In  the  mean  time  the  calcification  of  the  cartilage  matrix 
has  been  extending,  and  as  fast  as  the  network  of  calcified 
trabeculte  is  formed  it  is  invaded  by  the  mesenchyme, 
until  finally  the  cartilage  becomes  entirely  converted  into 
a  mass  of  spongy  bone  enclosed  within  a  layer  of  more 
compact  periosteal  bone. 

As  a  rule,  each  cartilage  bone  is  developed  from  a  single 
center  of  ossification,  and  when  it  is  found  that  a  bone  of 
the  skull,  for  instance,  develops  by  several  centers,  it  is 
to  be  regarded  as  formed  by  the  fusion  of  several  prima- 
rily distinct  bones,  a  conclusion  which  may  generally  be 
confirmed  by  a  comparison  of  the  bone  in  question  with 
its  homologues  in   the  lower  vertebrates.     Exceptions 
to  this  rule  occur  in  bones  situated  in  the  median  line  of 
the  body,  these  frequently  developing  from  two  centers 
lying  one' on  either  side  of  the  median  line,  but  such  cen- 
ters'are  usually  to  be  regarded  as  a  double  center  rather 
than  as  two  distinct  centers,  and  are  merely  an  expression 
of  the  fundamental  bilaterality  which  exists  even  in  me- 
dian structures. 

More  striking  exceptions  are  to  be  found  in  the  long 
b#wes  in  which  one  or  both  extremities  develop  from 
spt-cial  centers  which  give  rise  to  the  epiphyses  (Fig.  90, 
ep  ep'),  the  shaft  or  diaphysis  (d)  being  formed  from  the 
primarv  center.     Similar   secondary  centers   appear   in 
marked  prominences  on  bones  to  which  powerful  muscles 
are  attached  (Fig.  90,  a  and  b),  but  these,  as  well  as  the 
epiphvsial  centers,  can  readily  be  recognized  as  secondary 
from  th«-  fact  that  tliev  do  not  appear  until  much  later 
than  the  primarv  centers  of  the  bones  to  which  they  be- 
long     These  secondary  centers  give  the  necessary  firm- 
ness required  for  articular  surfaces  and  for  the  attachment 
of  musck•^  and.  at  the  same  time,  make  provision  for  the 
-rowth  in  length  of  the  bone,  since  a  plate  of  cartilage 


fc  ■'rrr^y^^^f^^^ 


THE    r.KDWTU    OK     UONES. 


179 


always  intervenes  between  the  epiphyses  and  the  diaphy- 
sis.  This  cartilage  continues  to  be  transformed  into  bone 
on  both  its  surfaces  by  the  extension  of  both  the  epiphy- 
sial and  diaphysial  ossification  into  it  and,  at  the  same 
time,  it  grows  in  thickness  with 
equal  rapidity  until  the  bone 
reaches  its  required  length,  where- 
upon the  rapidity  of  the  growth  of 
the  cartilage  diminishes  and  it 
gradually  becomes  completely  os- 
sified, uniting  together  the  epiphy- 
sis and  diaphysis. 

The  growth  in  thickness  of  the 
long  bones  is,  however,  an  entirely 
different  process,  and  is  due  to  the 
formation  of  new  layers  of  perios- 
teal bone  on  the  outside  of  those 
already  present.  But  in  connec- 
tion with  this  process  an  absorp- 
tion of  bone  also  takes  place.  A 
section  through  the  middle  of  the 
shaft  of  a  humerus,  for  example, 
at  an  early  stage  of  development 
would  show  a  peripheral  zone  of 
compact  bone  surrounding  a  core 
of  spongy  bone,  the  meshes  of  the 
latter  being  occupied  by  the  mar- 
row tissue.  A  similar  section  of 
an  adult  bone,  on  the  other  hand, 
would  show  only  the  peripheral 
compact  bone,  much  thicker  than 
before  and  enclosing  a  large  marrow  cavity  in  which 
no  trace  of  spongy  bone  might  remain.  The  differ- 
ence  depends  on  the  fact  that   as  the  periosteal  bone 


I"i(;.  '>0.  The  Osstfica- 
TioN  Centers  dk  the 
I'emuk, 

a  and  h,  Secondary  cen- 
ters for  the  great  and 
lesser  trochanters;  (/, 
(haphysis;  </',  iii)])er 
and  c//,  lower  epiphy- 
sis.— (Tcstiif.) 


i8o 


THE    DEVELOPMENT   OF   THE    HUMAN    BODV. 


increases  in  thickness,  there  is  a  gradual  absorption  of  the 
spongy  bone  and  also  of  the  earlier  layers  of  periosteal 
bone,  this  absorption  being  carried  on  by  large  multi- 
nucleated cells,  termed  osteoclasts,  derived  from  the  mar- 
row mesenchyme.  By  their  action  the  bone  is  enabled 
to  reach  its  requisite  diameter  and  strength,  without  be- 
coming an  almost  solid  and  unwieldy  mass  of  compact 
bone. 

During  the  ossification  of  the  cartilaginous  trabecule 
osteoblasts  become  enclosed  by  the  bony  substance,  the 


I^Vv^^^""  ^'"^r   '''V  ■^'■^""'■-«  -"^'«  '■""•«  V^EEKs;  /y.  THE  Samp 

The   heavy   black    line   represents   the   portion   of   hone   stained    l.v  the 
madder.      (Aftr,  llourms.) 

cavities  in  which  lliey  lie  formino  tlie  lacunrr  and  processes 
radiating  out  from  them  the  camiliculi,  so  characteristic  of 
bone  tissue.  In  the  growth  of  periosteal  bone  not  only  do 
osteoblasts  become  enclosed,  but  blood-vessels  also,  the 
HavnsuiH  canals  being  formed  in  this  wav,  and  around 
th,-se  lamella'  of  bone  are  deposited  by  the  enclosed  osteo- 
blasts to  form  Haversian  systems. 

That  the  absorption  „f  periosteal  bone  takes  place  during 
Kro„,  ,  ean  be  demonstrated  bv  taking  advantage  of  the  fact 
that    the  coloring  substance   madder,    when   consumed   with 


THE    VKRTElJk  K. 


I8l 


food  tinges  the  bone  being  formed  at  the  time  a  distinct  red. 
In  pigs  fed  with  madder  for  a  time  and  then  killed  a  section  of 
the  femur  shows  a  superficial  band  of  red  bone  (lig.  yi,  A). 
but  if  the  animals  be  allowed  to  live  for  one  or  two  nuuiths 
after  the  cessation  of  the  madder  feeding,  the  red  band  will  be 
found  to  be  covered  bv  a  laver  of  white  lione  varying  ui  thick- 
ness according  to  the  interval  elapsed  since  the  cessation  of 
feeding  (Fig.t;!,  H)  ;and  if  this  interval  amount  to  four  months, 
it  will  be  found  that  the  thickness  of  the  uncolored  bone  be- 
tween the  red  bone  and 
the  marrow  cavity  will 
have  greatly  diminished 
(Flourens). 

The  Development  of 
the  Skeleton. — Kmbryo- 
logically  considered,  the 
skeleton  is  composed  of 
two  portions,  the  axial 
5ifee/c/ow,  consistiiifr<rf-the 
skull,  the  verteljra?,  litts, 
and  sternum,  developing 
from  the_sclg{gtonj£5  of 
the  mesodermal  somites, 
and  the  appendicular 
skeleton,  which  includes 
the  pectoral  and  pelvic 
girdles  and  the  bones  of 
the    limbs,    and     which 

arises  from  the  mesenchyme  of  the  somatic  mesoderm. 
It  will  be  convenient  to  consider  first  the  development 
of  the  axial  skeleton,  and  of  this  the  differentiation  of  the 
vertebral  column  and  rib'^  may  first  be  discussed. 

The  Development  of  the  Vertebrae  and  Ribs.— The 
nR2sencIij2E'-'  tormed_from_Mie  sclen3toTmi_of_£ach  meso- 
dermic  somite  grows  inwar(!_t(j'v;ird  tlicjncdJan  lipcund 
forms  a  complete  investmeai  for  the  notochord,  and,  at 


Fici.     <'i.     TkAN>vHKsK     .Suction 

THKOl<.H        THH       l.NTEKVERTEBRAL 

Plate    of  the    First    Ckkvical 
Vektebka  or  A  Cauk  Kmbkvo  of 

8.H  MM. 

b(\  Intervenet<ral  i>lale ;  w*,  fxurtli 
ijiyotoine;  s,  liypocluirdal  bar;  .\/, 
spinal  accessory  nerve.      (1  ruriip.) 


I82 


THE    l)KVKI.O»'.\IKNT    OF    IIIK    111  \JAN    HODY, 


the  same  time,  sends  a  wing  dorsally  on  each  side  of  the 
medullary  canal,  so  that  this,  as  well  as  the  notochord, 
becomes  enclosed  by  a  series  of  mesenchyniatous  masses, 
each  of  which  is  separated  from  its  predecessor  and  suc- 
cessor by  a  plate  of  nu)re  densely  arranged  mesenchynia- 
tous cells  (Fig.  92,  be').     These  intervertebral  plates  are 


iM'i.  ').\     I.ONT.iTfDiNAi,  Section  thkoigh  the  Occii-ital  Ui  .,...>f  and 

Upper  Cekvicau  \'ertkbr.u  of  a  Cai.f  Kmbrvo  of  18. -^  mm. 
I>a.s,  Basilar  artery;  cli,  notodionl;  A.V'-<,  vortebral  centra  -  Ir^-*   inter- 
yertehral    disks;    ocr,    basiocdpital;     .SV'-*,     hypoeliordal    bars - 

(r  ri>riif>.) 

portions  of  the  intermuscular  septa  wliich  occupy  the  inter- 
vals between  adjacent  mesodermic  somites  and  are  formed 
of  cells  which  liave  wandered  from  the  anterior  and  poste- 
rior surfaces  of  the  somites.  At  first,  then,  the  invest- 
ment of  the  notochord  and  medullary  canal  is  by  a  series 
of    alternating    segmental   and    intersegmental    cellular 


idwJB 


THE    VEKTEHK.V.. 


183 


masses,  and  the  first  stage  in  the  development  of  the  verle 
bne  may  be  termed  the  celliilijr  stage. 

In  the  second  or  cartihiginous  stage  the  mesenchyme 
becomes  converted  into  cartilage  in  certain  definite  re- 
gions.    The  portions  of  sclerotomic  mesenchyme  which 
surround  the  notochord  become  chondrified  and  form  the 
vertebral  centra   (Fig.   93,  Kc),  these  structures  being 
therefore  segmental  and  corresponding  in  position  with  a 
pair  of  spinal  nerves,  myotomes  and  dermatomes.     The 
remaining  portions  of  each  vertebra  and  the  ribs  are  devel- 
oped in  the  intermuscular  septa,  and  are  therefore  inter- 
segmental in  position.     In  the  mesial  edge  of  each  sep- 
tum a  cartilaginous  bar  develops,  the  upper  part  of  which 
comes  into  contact  with   the  tip  of  the  corresponding 
bar  of  the  opposite  side  to  form  a  mural  arch,  while  the 
lower  end  becomes  connected  with  its  fellow  of  the  other 
side  b^  a  transverse  rod  of  cartilage  which  lies  below  the 
notochord  and  is  termed  the  hypochordal  bar  (Fig.  93.  Sc). 
Furthermore,  the  vcntraLfidge  of  each  intermuscular  sep- 
tum becomes  to  a  greater  or  less  extent  converted  into 
cartilage  to  form  a  rib.     The  neural  arches  1^1^ r  unite 
with  the_eiititra,  their  original  intersegmental  character 
being  thus  to  a  certain  extent  obscured,  but  the  ribs, 
which  typically  alternate  with  the  centra,  retain  their 
original  position.     The  hypochordal  bars  are  for  the  most 
part   merely   transitory   structures,   recalling   structures 
found  in  the  lower  vertebrates;  in  the  mammalia  they 
degenerate  before  the  completion  of  the  second  stage  of 
development,  except  in  the  case  of  the  atlas,  whose  devel- 
opment will  be  described  later.     The  cartilages  which 
form  the  neural  arches  are  at  first  simple  rodg,  but  later  a 
lateral  outgrowth  develops  on  each  to  form  a  transverse 
process  and  upon  the  ribs_i1    slighter  elfviilinn  fk:-^*4np^ 
to  form  the  tuberculum. 


mmm 


1.0 


I.I 


■  50 

|2.8 

^ 

Hi 

|a2 

11^ 

Hi 

IM 

|2.0 

l^ 

MICROCOPY  RESOLUTION  TEST  CHART 

NATIONAL  BUREAU  OF  STANDARDS 

STANDARD  REFERENCE  MATERIAL  1010a 

(ANSI  and  ISC  TEST  CHART  No.  2) 


1 84 


THE    DEVELOPMENT   OF    THE    HUMAN    BODY. 


The  portions  of  the  sclcrotomic  mesenchyme  which 
ijrew  up  around  the  medullary  canal  do  not  undergo 
chondrification  but  become  converted  into  dense  fibrous 
connective  tissue,  forming  the  supraspinous  and  inter- 
spinous  ligaments  and  the  ligan'enta  subflava.  The 
portions  of  the  intermuscular  septa  which  immediately 
surround  the  notochord  and  are  not  concerned  in  the 
formation  of  the  hypochordal  bars  are  converted  into 
fibrocartilaginous  tissue  forming  the  intervertebral 
disks  (Fig.  93, /c),  while  the  anterior  and  posterior  inter- 
vertebral ligaments  arise  from  portions  of  the  mesen- 
chyme from  which  the  centra  are  developed. 

Primarilv  the  notochord  traverses  the  entire  series  of 
centra  and  intervertebral  disks  as  a  continuous  rod,  but 
as  the  process  of  chondrification  proceeds  the  portions 
which  traverse  the  centra  gradually  become  encroached 
upon  and  eventually  are  completely  obliterated,  wliile 
in  the  intervertebral  disks  it  continues  to  grow  and  per- 
sists as  the  masses  of  pulpy  tissue,  one  of  which  occupies 
the  center  of  each  disk. 

The  mode  of  development  described  above  applies  to 
the  great  majority  of  the  vertebra,  but  some  departures 
from  it  occur,  and  these  may  be  conveniently  considered 
before  passing  on  to  an  account  of  the  ossification  of  the 
cartilages.  The  variations  affect  principally  the  extremes 
of  the  series.  Thus  the  posterior  vertebrae  present  a  re- 
duction of  the  parts  derived  from  the  intermuscular  sept?., 

the  neural_^irch£S_oLJi«L^^ 

fecbU-  de\ieloped,  while  inUi.e  coccvgcal  vertebra  they  are 
indicated  only  in  the  first.  In, the  first  cervjcaljLfillebra, 
the  atlas,  the  reverse  is  the  case,  for  the  entire  adult  ver- 
tebra  is  formed  from  the  intermuscuJaUi^Ttum ,  its  lateral 
masses  and  posterior  arch  being  the  neural  arches,  while 
its  anterior  arch  is  the  hypochordal  bar,  which  persists 


: 


Tilt    VEKTKIIK  K 


185 


in  this  vertebra  only.  A  ^ell-.levelopecl  centrum  .s  also 
formed,  however  (m,.  .„).  but  it  does  not  nn.te  w,th  tte 
parts  derived  fro.n  the  corresponding  mtennuscular  sep- 
C  bnt  during  its  ossif.ca.ion  unites  with  the  eentrun, 
o"  the  axis,  forndng  the  odontoid  process  of  that  vertebra. 
The  axis  consequently  consists  of  two  segn.ental  and  one 

intersegnK.ntal  portion,  while  the  atlas  consists  of  one 

intersetrnicntal  portion  only. 

The  extttit  to  which  ribs  arc  developed  in  connection 

with    thcTarious    vertebra,    also    varies    considerably 

Throui^hout  the  cervicaLregioii  thev_arcwrxJiort.  the 

upper  Wor^i^Uldn^noioil^^ 

."sses  with  the  tips_^fwhichUieirLcx^^  ^n 

early  stage.    Tnlfc-^^^jSne  or  six  vertebr^AIsMlV-cly 
igemtLalp^^ 

ari^nTS?dT?Tioirc^ameT'TirTT^^  the  ribs 

reaHTHiiirSf^KU^^tlirvelopment,  the  upper  eight  or  nine 
"tending  almost  to  the  mid-ventral  line    where  their 
extremities  unite  to  form  a  longitudinal  cartilaginous  bar 
from   which   the   sternum  develops    (see  p.    189).      The 
lower  three  or  four  thoracic  ribs  are  successively  shorter, 
however,  and  lead  to  the  condition  found  m  the  lumbar 
vertebra.,    where   they   are   again   greatly   reduced   and 
firmly  united  with  the  transverse  processes,  the  union 
being  so  close  that  only  the  tips  of  the  latter  can  be  distin- 
guished, forming  what  are  known  as  the  accessory  tuber- 
cles     Finally,  in  the  sacraWegion  the_ribs^nu:^ced  to 
short  flat  plates,  which  unite  together  to  form  the  latera 
tnasses  of   the  sacrum.       They  are  usually    developed 
only  in  connection  with  the  first  thjeejacralj^erteta. 
the  last  two  sacrals  and  the  coccygeals  having  no  ribs. 
16 


1 11 

l! 


W 


!i 


1 86 


TIIK    DKVELOl'MENT    OF    THE    IILMAN    liODV. 


The  limitation  of  the  ribs  to  the  three  anterior  sacral  verte- 
brae is  explained  by  the  fact  that  primarily  the  pelvic  girdle  is  in 
relation  only  to  the  last  two,  whose  ribs  are  consefiiientlv  sup- 
pressed. The  rib-bearing  sacral  vertebra.'  are  reallv  members 
of  the  lumbar  series  and  only  secondarily  come  into  relation 
with  the  iliac  bf)nes. 

The  third  sta,s^e  in  the  development  of  the  axial  skeleton 
begins  with  the  ossification  of  the  cartilages,  and  in  each 
vertebra  there  are  typically  as  many  primary  centers  of 
ossification  as  there  were  originally  cartili^es.  Thus, 
to  take  a  tlioracic  vertebra  as  a  type,  a  center  appears  in 
each  half  of  each  neural  arch  at  the  base  of  the  transverse 


Z=^e< 


V    )^J 


I'lc.  'M.  -.1,  A  Vkktebra  at  HiKTii;  />,  I.imhak  \'i;ktkhk  \  mkiwim; 
vSecondarv  Cknters  of  Ossification'. 

a,  Center  for  the  articular  pruccss;  c,  centrtitii;  <7,  hivvcr  cpiphvsi:!] 
I)late;  rn,  u])\n;T  epiphysial  j)latc;  va,  neural  areli ;  s,  center  lor 
s[)iiious  process;  /,  center  for  transverse  process,— (.S'u/j/irj.) 


process  and  gradually  extends  to  form  the  bony  lamina, 
pedicle,  and  the  greater  portion  of  the  transverse  and 
spinous  processes;  a  double  center  (see  p.  178)  gives  rise 
to  tlie  body  of  the  vertebra;  and  each  rib  ossifies  from  a 
single  center.  These  various  centers  appear  early  in  em- 
bryonic life,  but  the  complete  transformation  of  the  car- 
tilages into  bone  does  not  occur  until  some  ti  ne  after 
birtli,  -ach  vertebra  at  that  period  consisting  of  three 
pp  ls,  a  centrum  and  two  halves  of  an  arch,  separated  by 
u.iossificd  cartilage  (Fig.  94,  A).      At  al)out  pubert\  secoii- 


TllK    VEKTEUK.i;. 


187 


clary  centers  make  their  appearance;  one  appears  in  the 
cartilaj;e  which  still  covers  the  anterior  and  posterior  sur- 
faces of  the  vertebral  centra,  producing  disks  of  bone  in 
these  situations,  another  appears  at  the  tip  of  each  spinous 
and  transverse  process  (Fig.  94.  B),  and  in  the  lumbar 
vertebra  others  appear  at  the  tips  of  the  articulatmg 
processes.  The  epiphyses  so  formed  remain  separate 
until  growth  is  co!npleted  and  between  the  sixteenth  and 
twenty-fifth  years  utiite  with  the  bones  formed  from  the 


I'l,       'H  1      I  ITER  SUKfACE  OF   THE   I'IKST  SaCRAL  VERTEPRA,   AND  7^, 

'  Ventrau  View  of  the  Sacrum  showing   Primary  Centers  of 

(  )SSU"ICaTION'. 

(,  Centrum;  «<i,  neural  arch;  r,  rib  center.— (Sa/)/>e>'.) 


i 


primary  centers,  which  have  fused  by  this  time,  to  form 
a  single  vertebra. 

I'Uich  rib  ossifies  from  a  single  primary  center  situated 
near  the  angle,  secondary  centers  appearing  for  the  capit- 
ulum  and  tuberosity. 

lii  some  of  the  vertebrae  modifications  oi  this  typical 
mode  of  ossification  occur.  Thus,  in  the  upper  five  cervi- 
cal vertebra  the  centers  for  the  rudimentary  ribs  are  sup- 
pressed, ossification  extending  into  them  from  the  neural 


i88 


THE    OKVELOI'MENT    OK    THE    HLMAN    llODV. 


arch  centers,  and  a  similar  suppression  of  the  costal  cen- 
ter occurs  in  the  lower  lumbar  vertebra,  the  first  only 
developing  a  separate  rib  center.  Furthermore,  in  the 
atlas  a  double  center  appears  in  the  persisting  hypo- 
chordal  oar,  and  the  centrum  which  corresponds  to  the 
atlas,  after  developing  the  terminal  epiphy-ial  disks,  fuses 
with  the  centrum  of  the  axis  to  form  its  odontoid  process, 
this  vertebra  consequently  possessing,  in  addition  to  >.he 
typical  centers,  one  (double)  otlier  primary  and  two  sec- 
ondary centers.  In  the  sacral  region  the  typical  centers 
appear  in  all  five  vertebrae,  with  the  exception  of  rib 
centers  for  the  last  one  or  two  (iMg.  95)  and  two  ad- 
ditional secondary  centers  give  rise  to  plate-like  epi- 
physes on  each  side,  the  upper  plates  forming  the  articular 
surface  for  the  ilium.  At  about  the  twenty-fifth  year  all 
the  sacral  vertebra^  unite  to  foim  a  single  bone,  and  a 
similar  fusion  occurs  also  in  the  rudimentary  vertebra?  of 
the  coccyx. 

The  majority  of  the  anomalies  seen  in  the  vertebral  column 
are  due  to  the  imperfect  development  of  one  or  more  cartilages 
or  of  the  centers  of  ossification.  Thus,  a  failure  of  an  arch  to 
unit.''  with  the  centrum  or  even  the  complete  absence  of  an 
arch  or  half  an  arch  may  occur,  and  in  cases  of  spina  bifida 
the  two  halves  of  the  irches  fail  to  unite  dorsally.  Occa- 
sionally the  two  parts  of  the  double  center  for  the  body  fail 
to  unite,  a  double  body  resulting;  or  one  of  the  two  parts 
may  entirely  fail,  the  result  being  the  formation  of  only  one- 
half  of  the  body  of  the  vertebra.  Other  anomalies  result  from 
the  excessive  development  of  parts.  Thus,  the  rib  of  the 
seventh  cervical  vertebra  may  sometimes  remain  distinct  and 
be  long  enough  to  reach  the  sternum,  and  the  first  lumbar  rib 
may  also  fail  to  unite  with  its  vertebra.  On  the  other  hand, 
the  first  rib  is  occasionally  found  to  be  imperfect. 

The  Development  of  the  Sternum. — The  longitudinal 
bars  which  a  _■  formed  by  the  fusion  of  the  ventral  ends  of 
the  anterior  eight  or  nine  cartilaginous  ribs  represent  the 


»    = 
i 


THE    STERNUM. 


189 


future  sternum.  At  an  early  peru,.l  Uu-  two  .  .s  ^nc 
into  contact  anteriorly  and  fuse  together  iVvA-  9^.  and  at 
this  anterior  end  two  usually  indistinctly  separated  masses 
of  cartilage  are  to  be  observed  at  the  vicinity  of  the  points 
where  the  ventral  ends  of  the  cartilaginous  clavicles  artic- 
ulate These  are  the  epistcrnal  carhhwes  (em),  winch 
later  normallv  unite  with  the  longitudinal  bars  and  form 


p  ;i 


f 


F„;     96    -    IH.KMAT.CN   IK   THE   STERNt  M    ,.V    .SN    KmBRVO  OI-   ABOVT   3   CM. 

cl,  Clavic'e;  cm    epistcrnal  cartilages.-  (/v""*;*'.) 

part  of  the  manubrium  stcrni,  though  occasionally  they 
persist  and  ossify  to  form  the  ossa  supmstcnialia.  The 
fusion  of  the  longitudinal  bars  gradually  extends  oack- 
ward  until  a  single  elongated  pla^e  of  cartilage  results, 
with  which  the  seven  anterior  ribs  are  united,  one  or  two 
of  the  more  posterior  ribs  which  originally  took  part  in  the 
formation  of  each  bar  having  separated.     The  portions  of 


IQO 


TiiK  m:vi:i.oi'MENT  of  tiik  hi  man   iinnv. 


the  bars  fornic*!  by  tlu'sc  p<)stcrif)r  rihs  toiislittitr  (lie  insi 
form  process. 

Tlie  ossification  of  the  stermitii  (Vh^.  07)  partakes  to  a 
certain  extent  of  the  orii^inal  hihiteral  se.i,Mnental  ori<,Hn  of 
the  cartiluj^e,  hut  a  marked  condensation  of  tiie  centers  of 
ossification  also  occurs.  In  the  portion  of  the  cartiht^e 
wliich  Her,  below  the  junction  of  the  third  costal  cartilaj,'es 
a  scries  of  pairs  of  centers  appears  just  about  birth,  each 

center  probably  representing;  an 
epiphysial  center  f)f  a  correspond- 
ins;  rib.  Later  ihe  centers  of  each 
pair  fuse  and  the  simple  centers  so 
formed,  cxtcndini,^  throu,i,di  the 
cartilage,  eventually  unite  to  form 
the  greater  part  of  the  gladiolus. 
In  each  of  the  two  uj)permost  seg- 
ments, however,  but  a  single 
center  appears,  that  of  the  lower 
segment  uniting  with  the  more 
posterior  centers  and  forming  the 
upper  part  of  the  gladiolus,  while 
the  uppermost  center  gives  rise  to 
the  manubrium,  which  frequentlv 
persists  a-^  a  distinct  bone  united  to 
the  gladiolus  by  a  hinge-joint. 

A  failure  of  the  cartilaginous  bars 
to  fuse  produces  the  condition  known 
as  clcjf  slciniim,  or  if  the  failure  to  fuse  affects  onlv  a  por- 
tion of  the  bars  there  resuUs  a  perforated  sternum.  A  per- 
foration or  notching  of  the  ensiforni  cartilage  is  of  frequent 
occurrence  owing  to  this  being  the  region  where  the  fusion 
of  the  bars  takes  place  last. 

The  suprasternal  bones  are  the  rudiments  of  a  large  bone, 
the  episternuni.  situated  in  front  of  the  manubrium  in  the 
lowest  mammalia  and  reptilia.  It  furnishes  the  articular  sur- 
faces for  the  clavicles  and  is  possibly  formed  by  a  fusion  of  the 


Imc.     97. — Ster.nl'.m     ok 
.V  K  \v  -BORN     Child, 

SHOWI.\G     CENTKKS    OF 
OsSIF'C.\TION-. 

/  to  IV/,  Costal  cartila<,'i'S. 


ki. 


TlIK    SkUI.I.. 


\()l 


vcntr.l  .nds  of  the  carlilagcs   which   r.,,r..cnt    '^^^J^l^'r; 
lunce  ils  uppouraiuo  as  a  pair  nl  h.MKS  in  the  nulmuntan 

Cnil(lilil)Il. 

The  Development  of  the  Skull.-I.ittk'  is  as  yet  known 
cspeciallv  in  the  human  embryo,  ccncernini;  the  or.g.n  ot 
the  mesenehvme  from  which  the  mammalian  skull  is  de- 
velope<l.  vet',  since  there  is  probably  a  continuation  for- 
ward into  the  cranial  region  of  the  series  of  mesodernnc 
?omites,  it  is    suppos- 
able  that  these  furnish 
the    TP'     "''  vme   for 
the  sk  as  the 

morel      •  somites 

furnish  l    for    the 

vertebra'. 

In  the  earliest  stages 
the  human  skull  is  rep- 
resented by  a  continu- 
ous mass  of  mesen- 
chyme which  invests 
the  anterior  portion  of 
the  notochord  and  ex- 
tends forward  beyond 
its  extremity  into  the 
nasal  region,  forming  a 

core  for  the  frontonasal  process  (see  p.  104).  From  each 
side  of  this  basal  mass  a  wing  projects  dorsally  to  enclose 
the  anterior  portion  of  the  medullary  canal  which  will 
later  become  the  cerebral  part  of  the  central  nervous  sys- 
tem No  indications  of  a  segmental  origin  are  to  be 
found  in  this  mes  nchyme;  as  stated,  it  is  a  continuous 
mass,  and  this  is  likewise  true  of  the  cartilage  which  later 

develops  in  it.  ,.       r 

The  chondrification  occurs  first  along  the  median  line 


Pui  98— Reconstruction  OF  TiiE  Chon- 

DROCRANUM  OF  AN  KmBRYO  OF   14  MM. 

m-  Alisphenoid;  bo,  basioccipilal;  ba, 
hasisplienoid;  co,  exoccipital;  m, 
Meckel's  cartilage;  os,  orhitosphenoid; 
p,  periotic;  ps,  presjihcnoi.l ;  .so,  sella 
turcica ;  s,  supraoccipital. — {Levi.) 


I  i 


192 


TMK    DF.VEI.OPMENT    OI"    TlIK    IIIMAN    IIOMV, 


in  what  will  be  the  occipital  and  splienoidal  rej^ions  of  the 
skull  (iMjti;.  98)  atul  thence  ijradually  extends  forward  into 
the  ethnioidjil  re>,don  and  to  a  certain  e  tent  dorsally  at  the 
sides  and  behind  into  the  re,t,dons  later  occupied  \>y  the 
winjjs  of  the  sphenoid  (as  and  os)  and  the  scjuatnous  portion 
of  the  occipital  (s).  \o  cartilaj^e  develops,  however, 
in  the  rest  of  the  sides  or  in  the  roof  of  the  skull,  but  the 
mesenchyme  of  these  rei,dons  becomes  converted  into  a 
dense  membrane  of  connecti\  e  tissue.  While  the  chondri- 
fication  is  proceedins;  in  the  regions  mentioned,  the  mesen- 
chyme which  encloses  the  internal  ear  becomes  converted 
into  cartilage,  forming  a  mass,  the  pcriolic  capsule  fFig. 
98,  p),  wedged  in  on  either  side  between  the  occipital  and 
sphenoidal  regions,  with  wliich  it  eventually  unites  to 
form  a  continuous  chondrocranium,  perforated  by  fora- 
mina for  the  passage  of  nerves  and  vessels. 

The  posterior  part  of  the  basilar  portion  of  the  occipital 
cartilage  presents  certain  peculiarities  of  development. 
In  calf  embryos  there  are  in  this  region,  in  very  early 
stages,  four  separate  condensations  of  mesoderm  corre- 
sponding to  as  many  mesodermic  somites  and  to  the  three 
roots  of  the  hypoglossal  nerve  together  with  the  first 
cervical  or  suboccipital  nerve  (Froriep)  (Fig.  99).  These 
mesenchymal  masses  in  their  general  characters  and  rela- 
tions resemljle  vertebral  centra,  and  there  are  <i^ood  reasons 
for  believing  that  they  represent  four  vertebrae  which,  in 
later  stages,  are  taken  up  into  the  skull  region  and  fuse 
with  the  primitive  chondrocranium.  In  the  human  em- 
bryo they  are  less  distinct  than  in  lower  mammals,  but 
since  a  three-rooted  hypoglossal  and  a  suboccipital  nerve 
also  occur  in  man  it  is  probable  that  the  corresponding 
vertebrae  are  also  represented,  ^ideed,  confirmation  of 
their  existence  may  be  found  in  lii^  fact  that  during  the 
cartilaginous  stage  of  the  skull  the  anterior  condyloid 


t3 


Till     ^kll.l.. 


9.^ 


foratnina  art-  dividcil  into 
inous  parlitioiiN  whicli   st 
hyi)<),i,'l()ssal     mrvf.       It 
seems  certain    from    the 
evidence    derived     from 
cmbryoloijy  and  compar- 
ative anatomy  that   the 
human  skull  is  composed 
of    a    primitive     unsei, 
mental    clu>f     'jcrai'ium 
plus   four    vei     bra',  tin- 
latter  l)ein,ii;  added  (o  and 
incorporated     with     the 
occipital   portion  of  the 
chondrocranium. 

iCmphasis     must      be 
laid  upon  the  fact   that 
the  cartiiaijinous  portion 
of    the  skull  forms  onh- 
the  base  and  lower  por- 
tions of  the  sides  of  the 
cranium,  its  entire  roof, 
as  well  as  the  face  region, 
showing  no  indication  of 
cartilage,     the      mesen- 
chyme  in  these  regions 
being  converted  into  fi- 
brous connective  tissue 
whicli.  especially  in  the 
cranial    region,    assumes 
the  form  of  a  dense  mem- 
brane. 

But  in  addition  to  the 
chondrocranium  and  the 
vertebrae      incorporated 


three  portions  l)y  two  cartilag- 
l)arate   the  three  roots  of   the 


Vic.  ')9.     1'"k(intai,  Sectton  turocch 

THE    OCCirlTAl,    AM)    riM'EK    CERVI- 
CAL Re<;i(>ns  or  a  Cai.k  I{NrBRv()  ov 

8.7   MM. 

(j(  and  .h',  IntiTvcrU'ljral  arteries;  Ih\ 
first  eervical  intervertebral  plate; 
1)0,  suboccipital  intervertebral  plate  ; 
c'--,  eervical  nerves;  cit,  notoeliord ; 
K,  vertebral  centriim  ;  w'-'',  occipi- 
tal myotomes;  m*-',  cervical  n!y•^- 
toines;  o'-''  roots  of  '.ypoglossal 
nerve;  ij.  '■  ,,ular  vein;  x  and  xi, 
vagus  ar..^  spinal  accessory  nerves. 
—  ([•  roriep.) 


l<)4 


nil      DKVKKtl'MFSr    1)1"    nil".    niMAN     i.(l|i\. 


witli  it.otlur  cartila.uiti'Mis  ik-intnts  tiiUt  into  the  com- 
position of  tlir  skull.  The  nusenchynii'  whicli  ocrupiis 
the  iixis  of  tach  I.  I'u-hial  arch  undirj^oi-s  more  or 
less  complete  ehondrilieation,  eartilaK'inons  l)ars  being 
so  fornad,  certain  of  which  enter  into  very  close  rela- 
tions with  the  skull.  It  has  been  seen  (p.  97)  that 
each  half  of  the  first  arch  gives  rise  to  a  nia.xillary  process 
whicli  grows  forward  and  ventrally  to  form  the  anterior 
boundary  of  the  tnouth,  while  the  remaining  portion  of  the 
arch  forms  the  mandibular  process.  Cartilage  appears  in 
the  posterior  or  dorsal  part  of  each  maxillary  process,  and 
the  rod  so  formed  applies  itself  b\  its  ventral  end  to  the 

under  surface  of  the 
sphenoid  region  of 
the  chondrocranium, 
forming  the  cartilagi- 
nous Uernal  ptery- 
goid plate.  The  whole 
of  the  axis  of  the 
mandibular  process, 
on  the  other  hand, 
becomes  chondrified, 
forming  a  rod  known 
as  Meckel's  cartilage, 
and  this,  at  its  dorsal 
end,  comes  into  relation  with  the  periotic  capsule,  as 
does  also  the  dorsal  end  of  the  cartilage  of  the  second 
arch.  In  the  remaining  three  arches  cartilage  forms 
only  in  the  ventral  portions,  so  that  their  rods  do  not 
come  into  relation  with  the  skull,  though  it  will  be  con- 
venient to  consider  their  further  history  together  with 
that  of  the  other  branclr  arch  cartilages.  The  ar- 
rangement of  these  cartilages  is  shown  diagrammatic- 
ally  in  Fig.  100. 


I'lo.    KJO.  — I)i.\<;k.\m    showtni.  tiu:   Fivk 

liUANCIIIAL   C.\KTII.Ai;!:s,     /    to    l". 

/',  Inttriial  ptery;;i)i(l  ])V'ici-ss  c.f  ilie  splu-- 
iiuid;  At,  atlas;  .l.v,  axis;  .<,  third  cirvi- 
cal  vfrtebra. 


Till     >^kUI,l,. 


•95 


\\\  till-  (»ssirKMti()n  of  tlusi-  various  parts  tlirir  cattyo 
rii's  of  Itotus  ari'  foniiod:  (i^     artilai^i-  hoius  fornad  in 
the  choiKlroiraiiiiitii,  (2)  iiu'tubraiic  hotus,  and  (3)  inir- 
tilatjc'  hoiKs  (It  \ rlopins;  froju  tlie  cartilams  of  the  bran- 
thial  arclics.     Tlif  hoJRs  luloiiijinK  to  c-acli  of  tht'SC  c\.t- 
I'Uorit's  arc  primarily  (|uitc-  distinct  irom  one  another  and 
from  those  of  the  other  .lironps.  l)ut  in  the  liunian  skull  a 
verv  considerable  amount  of  fusion  of  the  ])rimary  boms 
takes  place,  and  elements  belon.i;inn  to  two  or  even  to  all 
three  categories  may  tuiite  to  form  a  sinidc  Ixme  of  the 
adult  skull.     In  a  certain  regicm  of  the  clun.drocrani      ' 
also  and  in  one  of  the  branchial  arches  the  origi'ial  ca: 
lage  bone  becomes  ensheathed  by  membraue  ^^oul-  and 
eveiiluallv  (lisai)pears  completely,  so  t'-  t  the  cu   I't  bone, 
althou-h  represented  bv  a  cartilage,  is      .Uy  a  membrane 
l)()iie.     And,  indeed,  tnis  process  has  proceeded  so  far  in 
certain  portions  of  the  branchial  arch  skeleton  that  the 
original  cartilaginous  rei)resentatives  are  no  longer  de- 
veloped, but  the  bones  arc  deposited  directly  in  connecti  ,e 
tissue.     These    various    modifications    interfere    greatly 
with  the  precise  application  to  the  human  skull  of  the 
classification   of  bones  into   the   three  categories  given 
above,  and  indeed  the  true  significance  of  certain  of  the 
skull  bones  can  only  be  perceived  by  comparative  studies. 
Nevertheless  it  seems  advisable  to  retain  the  classification, 
indicating,  where  necessary,  the  confusion  of  bones  of  the 
various  categories. 

The  Ossification  of  the  Chondrocranium.  The  ossifica- 
tion of  the  cartilage  of  the  occipital  region  results  in  the 
formation  of  four  distinct  bones  which  ev  n  at  birth  are 
separated  from  one  another  by  bands  of  cartilc  je.  The 
portion  of  cartilage  lying  in  front  of  the  foramen  magnum 
ossifies  to  form  a  basioc-ipital  bone  (Fig.  loi,  bo),  the  por- 
tions on  cither  side  of  this  give  rise  to  the  two  exo,  ipitals 


\()Cy 


THE    nEVEI.OPMENT   OF   THE    HUMAN    liODY. 


(eo),  wliicli  hear  the  condyles,  and  the  portion  above  the 
foramen  produces  a  supmoccipital  (so),  which  represents 
the  part  of  the  squamous  portion  of  the  adult  bone  lying 
below  the  superior  nuchal  line.  All  that  portion  of  the 
bone  which  lies  above  that  line  is  composed  of  membrane 
bone  which  owes  its  origin  to  the  fusion  of  two  or  some- 
times four  centers  of  ossification,  appearing  in  the  mem- 
branous roof  of  the  embryonic  skull.  The  bone  so  formed 
{ip)  represents  the  interparietal  of  lower  vertebrates  and, 

at  an  early  stage,  unites 
with  the  supraoccipital, 
although  even  at  birth 
an  indication  of  the  line 
of  union  of  the  two  parts 
is  to  be  seen  in  two  deep 
incisions  at  the  sides  of 
the  bone.  The  union  of 
the  exoccipitals  and  su- 
praoccipital takes  place 
in  the  course  of  the  first 
or  second  year  after 
birth,  but  the  basioccipi- 
tal  does  not  fuse  with 
the  rest  of  the  bone  until 
the  sixth  or  eighth  year. 
It  will  be  noticed  that 
no  special  centers  occur 
for  the  four  occipital  ver- 
tebrae, these  structures  having  become  completely  incor- 
porated in  the  chondrocranium,  and  even  the  cartilagi- 
nous partitions  which  divide  the  anterior  condyloid  for- 
amen usually  disappear  during  the  process  of  ossification. 
In  the  sphenoidal  region  the  number  of  distinct  bones 
which  develop  is  much  greater  than  in  the  occipital  region. 


Fig. 


OF 


101. — {)ceii'iT.\L     Bone 
Fetis  .\t  Term. 
/)(),    Hasioccipital;    co,    exoccijiital ;    if>, 
inter])arietal;  .so,  supraoccipital. 


THE   SKULL. 


197 


In  the  first  place,  the  basal  portion  of  the  cartilage  ossifies 
to  form  two  bones,  an  anterior  or  presphenoid  and  a  poste- 
rior or  basisphenoid  (Fig.  102,  b),  and  on  each  side  of  each 
of  these  an  ossification  appears  giving  rise  to  two  lesser 
wings  or  orbitosphenoids  {os)  and  two  greater  wings  or 
alisphenoids  (as),  and  an  additional  center  appears  on 
each  side  of  the  basisphenoid  to  form  the  hngula  (1).     in 
the  course  of  the  third  month  the  lingulae  fuse  with  the 
basisphenoid,   the  orbitosphenoids  unite  with  the  pre- 
sphenoid at  about  the  sixth  month,  and  a  little  later  the 
presphenoid  and  basisphenoid  unite,  the  fusion  of  the 
alisphenoids  with  the  basisphenoids  not  taking  place  until 
after  birth.     The  centers  which  give  rise  to  the  alisphe- 


i;„.    io->  —Sphenoid  Bone  from  Embryo  ok  .^i  to  4  Months. 
The   m'rts  wliich  are  still  cartilaginous  are  represented  in  black      .i^, 
AUsphenoid;    b,    basisphenoid;    /.    Hngula;    os,    orbitosphenoul ,    f. 
internal  pterygoid  plate.— {Sap pey.) 

noids  extend  into  the  external  pterygoid  plates,  but  the 
internal  plates  (p)  are  formed  by  membrane  bone  which 
encloses  and  eventually  replaces  the  pterygoid  cartilage 
derived  from  the  first  branchial  arch.  It  seems  probable 
that  the  upper  anterior  angle  of  the  alisphenoids  arises 
from  a  special  ossification  developing  in  membrane  in  this 

region.  ,    ,       ,       1 

The  cartilage  of  the  ethmoidal  region  of  the  chondro- 
cranium  forms  somewhat  later  than  the  other  portions 
and  consists  at  first  of  a  stout  median  mass  projecting 
downward  and  forward  into  the  frontonasal  process  (Fig. 


II 


wmm 


198 


THE    DEVELOl'MKNT   OF    THE    HUMAN    liODY. 


^03.  Ip)  and  two  lateral  masses  (Im),  situated  one  on  either 
side  in  the  mesenchyme  on  the  outer  side  of  each  olfactory 
pit.  Ossification  of  the  lateral  masses  or  cdcthmoids  be- 
gins relatively  early,  but  it  appears  in  the  upper  part  of  the 
median  cartilage  only  after  birth,  producing  the  crista 
galli  and  the  perpendicular  plate,  which  together  form 
what  is  termed  the  mesethmoid.  When  first  formed, 
these  three  bones  are  quite  separate  from  one  another,  the 
olfactory  and  nasal  nerves  passing  down  between  them  to 

the  olfactory  pit,  but  later 
bony  trabecule  begin  to  ex- 
tend across  from  the  junction 
between  the  crista  galli  and 
perpendicular  plate  to  the 
upper  part  of  the  ectethmoids 
and  eventually  form  a  fenes- 
trated horizontal  lamella,  the 
cribriform  plate. 

The  lower  part  of  the  me- 
dian cartilage  does  not  ossify, 
but  a  center  appears  on  each 
side  of  the  median  line  in  the 
mesenchyme  behind  and  be- 
low its  posterior  or  lower  bor- 
der. From  these  centers  two 
vertical  bony  plates  develop 
which  unite  by  their  median  surfaces  below,  and  above 
invest  the  lower  border  of  the  cartilage  and  form  the 
vomer.  The  portion  of  the  cartilage  which  is  thus  invested 
undergoes  a  certain  amount  of  resorption,  but  the  more 
anterior  portions  persist  to  form  the  cartilaginous  septum 
of  the  nose.  The  vomer,  consequently,  is  not  really  a 
portion  of  the  chondrocranium,  but  is  a  membrane  bone; 
its  intimate  relations  with  the  median  ethmoidal  carti- 


Fia.  103.  —Anterior  Portion 
OF  THE  Base  op  the  Skull 
OF  A  6   TO   7   Months'    I^m- 

BRYO. 

The  shaded  parts  represent  car- 
tilage, cp,  Crihriforin  i)late; 
Im,  lateral  mass  of  the  cth- 
inoid  ;  Ify,  perpendicular  plate ; 
of,  optic  '/ramen;  os,  orbi to- 
sphenoid.  -(.l//tr  -von  Spec.) 


THE    SKULL. 


199 


lage,  however,  make  it  convenient  to  consider  it  in  this 

place.  , 

When   first   formed,  the   ectethmoids   are   masses   of 
spongy  bone  and  show  no  indication  of  the  honeycombed 
appearance  which  they  present  in  the  adult  skull.     This 
condition  is  produced  by  the  absorption  of  the  bone  of 
each  mass  by  evaginations  into  it  of  the  mucous  mem- 
brane lining  the  nasal  cavity.     This  same  process  also 
brings  about  the  formation  of  the  curved  plates  of  bone 
which  project  from   the  inner   surfaces  of  the  lateral 
masses  and  are  known  as  the  superior  and  middle  turbm- 
ated   bones.     The    inferior   and    sphenoidal   turbmatcd 
bones  are  developed  from  special  centers  but  belong  to  the 
same  category  as  the  others,  being  formed  from  portions 
of  the  lateral  ethmoidal  cartilages  which  become  almost 
separated  at  an  earl v  stage  before  the  ossification  has  made 
much  progress.     Absorption  of  the  body  of  the  sphenoid 
bone  to  form  the  sphenoidal  cells,  of  the  frontal  to  form  the 
frontal  sinuses,  and  of  the  maxillary  to  form  the  antrum 
of  Highmore  is  also  produced  by  outgrowths  of  the  nasal 
mucous  membrane,  all  these  cavities,  as  well  as  the  eth- 
moidal cells,  being  continuous  with  the  nasal  cavities  and 
lined  with  an  epithelium  which  is  continuous  with  the 
mucous  membrane  of  the  nose. 

In  the  lower  mammalia  the  erosion  of  the  mesial  surface 
of  the  ectethmoidal  cartilages  results,  as  a  rule,  in  the  forma- 
tion of  five  turbinated  plates,  while  in  man  but  three  are 
usually  recognized.  Not  infrequently,  however,  the  human 
middle  turbinated  bone  shows  indications,  more  or  less  marked, 
of  a  division  into  an  upper  and  a  lower  portion,  which  corre- 
spond to  the  third  and  fourth  bones  of  the  typical  mammalian 
arrangement.  Furthermore,  at  the  upper  portion  of  the  nasal 
wall  in  front  of  the  superior  turbinate,  a  slight  elevation, 
termed  the  agger  nasi,  is  alwavs  observable,  its  lower  edge 
being  prolonged  downward  to  form  what  is  termed  the  uncinate 
process  of  the  ethmoid.  This  process  and  the  agger  together 
represent  the  first  turbinate  of  the  typical  arrangement,  to 
which,  therefore,  the  human  arrangement  may  be  reduced. 


Ill 

4 


200 


THE    DEVELOI'MENT   OF    THE    HUMAN    BODV. 


A  number  of  centers  of  ossification— the  exact  number 
IS  yet  uncertain— appear  in  the  periotic  capsule  during 
the  later  portions  of  the  inth  month,  and  during  the  sixth 
month  these  unite  together  to  form  a  single  center  from 
which  the  complete  ossification  of  the  cartilage  proceeds 
to  form  the  petrous  and  mastoid  portions  of  the  temporal 
bone  (Fig.  104,  p).  The  mastoid  process  does  not  really 
form  until  several  years  after  birth,  being  produced  by  the 
hollowing  and  bulging  out  of  a  portion  of  the  petrous  bone 

by  outgrowths  from  the  lin- 
ing membrane  of  the  middle 
ear.      The  cavities  so  formed 
are    the    mastoid    cells,    and 
their  relations  to  the  middle- 
ear  cavity  are  in  all  respects 
similar  to  those  of   the  eth- 
moidal and  sphenoidal  cells  to 
the   nasal   cavities.     The   re- 
maining portions  of  the  tem- 
poral bone  are  partly  formed 
by      membrane      bone      and 
partly     from    the     branchial 
arch  skeleton.    An  ossification 
appears    in     the     membrane 
which  forms  the  side  of  the 
skull  in  the  temporal  region  and  gives  rise  to  a  squamosal 
bone  (s),  which  later  unites  with  the  petrous  to  form  the 
squamosal  portion  of  the  adult  temporal,  and  another 
membrane  bone,  the  tympanic  (/),  develops  from  a  center 
appearing  in  the  mesenchyme  surrounding  the  external 
auditory  meatus,  and  later  also  fuses  with  the  petrous  to 
form  the  floor  and  sides  of  the  external  meatus,  ..giving 
attachment  at  its  inner  edge  to  the  tympanic  membrane. 
Finally,  the  styloid  process  is  developed  from  the  upper 


Fig.  104.— The  Temporai,  Hone 
.\T  Birth.  The  Stvi.oiu 
Process  .\ni)  Auditory  Os- 
sicles ARE  -NOT  Repre- 
sented 

/>,  Petrous  bone;  s,   S(jiiamnsal; 
/,  tympanic.  -(/'(i(V((T.) 


TIIK    SKU!,I.. 


20I 


U  i 


part  of  the  second  branchial  arch,  whose  history  will  be 

considered  later.  ..,     ■  •     ♦),,.     -imidro- 

The  various  ossifications        .ch  lorm  m  the      londro 

cranium  and  the  portions  ot  the  adult  skull  whu.  repre- 
sent them  are  sho^vn  in  the  following  table: 

I'AR.S  OF  Am  IT  Skiu.i.. 

Basilar  process. 
Condyles. 

Stiuamous  portion  above  su- 
jienor  nuchal  line. 

Hcjuy. 

Greater  wings  and  external 
pterygoid  plates. 

I.esscr  wings. 
r  I.omina  perpendiculans. 
\  Crista  galli. 
(.Nasal  septum, 
f  J<ateral  masses. 
I  Superior  turbinated. 
(Middle  turbinated. 


RkGION  ok  OSSIITCATIONS. 

Qhondkocranhm. 

(  Hasioccipilal 

Occipital.   Hx..ccipitals 

(.Supraoccipital 

Basisplienoid 
I'resphcnoid 

Lingulx 
Alisplicnoi'ls 


Sphe.ioidal, 


Ethmoidal, 


Orbitosphenoids 
Mesethmoid 

Ectethnioids 

Inferior  turbinated 
.  Sphenoidal  turbinated 


(  Petrous. 
\  Mastoid. 


Periotic  capsule,   

The  Membrane  If^ones  of  the  SkuU.-In  the  membrane 
forming  the  sides  and  roof  of  the  skull  in  the  second  stage 
of  its  development  ossifications  appear,  which  give  n.e, 
in  addition  to  the  interparietal  and  squamosal  bones 
already  m.-ntioued  in  connection  with  the  occipital  and 
tlporal,  to  the  panetals  and  frontal.     Eaeh  of  theformer 
bones  develops  from  a  sirigle  center,  while  the  front,   is 
formed  from  two  centers  situated  symmetrically  on  each 
side  of  the  median  line  and  eventually  fusing  completely 
to  f.  :m  a  single  bone,  although  more  or  less  distinct  indi- 
cations of  a  median  suture,  the  metopic,  are  not  infre- 
quently present. 

Furthermore,  ossifications  appear  in  the  mesenchyme 
of  the  facial  region  to  form  the  nasal,  lachrymal,  and 
17 


202 


THE    OEVELOPMENT    OF    THE    HUMAN    HOIjY. 


malar  bones,  the  first  two  arising  from  single  centers  of 
ossification,  while  each  malar  possesses  three  centers 
which  early  unite,  though  occasionally  one  or  more  of 
their  lines  of  union  may  persist,  producing  a  divided 
malar. 

The  vomer,  which  has  already  been  described,  belongs 
also  strictly  to  the  category  of  membrane  bones,  as  do  also 
the  maxillse and  palatines;  these  latter,  however,  primarily 
belonging  to  the  branchial  arch  skeleton,  with  which  they 
will  be  considered. 

The  purely  membrane  bones  in  the  skull  are,  then,  the 
following : 


Interparietals,    Part    of    squamous    portion    of 

occipital. 
S<iuamosals Squamous     portions     of     ten. 

porals. 

Tympanies, Tynqjanic  plates  of  tenq)orals. 

Parietals. 

Frontal. 

Nasals. 

Lachrymals. 

Malars. 

Vomer. 

The  Ossification  of  the  Branchial  Arch  Skeleton. — It  has 

been  seen  (p.  194)  that  a  cartilaginous  bar  develops  only 
in  the  dorsal  portion  of  the  maxillary  process  of  the 
first  branchial  arch.  This  cartilage  becomes  invested  by 
membrane  bone  which  gradually  replaces  the  cartilage  and 
eventually  fuses  with  the  sphenoid  bone  to  form  its  in- 
ternal pterygoid  plate.  In  the  more  ventral  portions  of 
the  maxillary  process,  1-  ever,  no  cartilaginous  skeleton 
forms,  but  two  membr.ae  bones,  the  palatine  and  max- 
illa, are  developed  in  it,  their  cartilaginous  representatives, 
which  are  to  be  found  in  lower  vertebrates,  having  been 
suppressed  by  a  condensation  of  the  development.     The 


T!1K    liK 


ANCHIAI.    AKCII    SKEI.KTON. 


203 


i 

1 

% 

t 

! 

-i 

i 
■ 

i 

! 


palatine  bone  dcvelons  from  a  single  center  of  ossiHca  .on, 
hut  for  each  maxilla  no  less  Uu-.n  live  cet.ters  have  been 
described  (Fig.  105).     One  of  these  gives  rise  to  so  much 
of  the  alveolar  border  of  the  bone  as  contains  the  bicuspid 
and  molar  teeth ;  a  second  forms  the  nasal  process  and  the 
part  of  the  alveolar  border  which  contains  the  canme 
tooth;  a  third  the  portion  which  contams  the  incisor 
teeth ;  while  the  fourth  and  fifth  centers  lie  above  the  first 
and  give  rise  to  the  inner  and  outer  portions  of  the  orbu  al 
plate  and  the  body  of  the  bone.     The  first,  second  fourth 
and  fifth  portions  early   unite    together,  but  the  third 
center,  which  really  lies  ia 
the   ventral     part    of    the 
fronto-nasal     process,     re- 
mains   separate    for    some 
time ,  forming  what  is  termed 
the  premaxilla,  a  bone  which 
remains    permanently    dis- 
tinct in  the  majority  of  the 
lower  mammals. 


Vic.  105.— DlAC.HAM  OF  THE  OS- 
SII'ICATIOS?  jF  which  THE  .MAX- 
ILLA IS  CuMPOSEO,  AS  SEEN  FROM 
THE        f^'TER      SlRKACE.         ThE 

Arrow  assES  throuoh  the 
Infra  jITAl  Canal.-  {From 
von  Spec,  alter  Sappey.) 


Since  the  condition  known 
as  cleft  palate  results  from  a 
failure  of  the  maxillar>'  pro- 
cess  to  unite  with  the  fronto-nasal  process  (see  p.  105),  and 
since  the  premaxilla  develops  in  the  latter  and  the  maxilla  m 
the  former,  the  cleft  passes  between  these  two  bones  and  pre- 
vents their  union. 

The  upper  end  of  Meckel's  cartilage  passes  between  the 
tympanic  bone  and  the  outer  surface  of  the  periotic  cap- 
sule and  thus  comes  to  lie  apparently  within  the  tympanic 
cavity  of  the  ear;  this  portion  of  the  cartilage  divides  into 
two  parts  which  ossify  to  form  two  of  the  bones  of  the  mid- 
dle ear,  the  malleus  and  incus,  a  description  of  whose 
further  development  may  be  postponed  until  a  later  chap- 


204 


THK    DKVELOPMENT    OK    TIIK    HUMAN    HODV 


ter.  The  lower  half  of  the  ventral  portion  of  the  eartilage 
becomes  completely  invested  i)y  a  number  of  flat  mem- 
brane bones,  which  fuse  together  so  as  to  enclose  the  car- 
tilage together  with  the  vessels  and  nerve  (inferior  dental) 
which  lie  beside  it.  Later  the  cartilage  disappears  and  a 
canal  containing  the  vessels  and  nerve  is  left  traversing 
the  fused  bones  which  represent  the  horizontal  ramus  and 
the  lower  part  of  the  vertical  ramus  of  the  mandible.  The 
upper  part  of  the  vertical  ramus  is  formed  of  membrane 
bone  also,  but  in  this  case  the  bone  lies  entirely  on  the  outer 
side  of  the  cartilage,  whence  the  position  c^  the  dental  fora- 
men on  the  inner  surface  of  the  ramus.  The  upper  half 
of  the  ventral  portion  of  the  cartilage  which  corresponds 
to  this  upper  part  of  the  ramus  undergoes  a  degeneration, 
forming  the  spheno-mandibular  ligament,  and,  in  the  later 
stages  of  development,  cartilage  develops,  quite  inde- 
pendently of  the  original  Mcckelian  cartilage,  at  the  sym- 
physis, the  articular  surface,  the  coronoid  process  and  the 
angle,  and  may  undergo  ossification,  becoming  eventually 
united  to  the  membrane  bone ;  these  cartilages  are  to 
be  regarded  as  secondary  epiphysial  cartilages. 

The  upper  part  of  the  cartilage  of  the  second  branchial 
arch  also  lies  within  the  tympanic  cavity  and  ossifies  to 
f  rm  the  stapes,  while  the  portion  of  the  cartilage  imme- 
diately ventral  to  this  ossifies  as  the  styloid  process  of  the 
temporal  bone.  The  succeeding  moiety  of  the  cartilage 
undergoes  degeneration  to  form  the  stylohyoid  ligament, 
while  its  most  ventral  portion  ossifies  as  the  lesser  cornu  of 
the  liyoid  bone.  The  great  variability  which  may  be  ob- 
served in  the  length  of  the  styloid  processes  and  of  the 
lesser  cornua  of  the  liyoid  depends  upon  the  extent  to 
which  the  ossification  of  the  original  cartilage  pioceeds, 
the  length  of  the  stylohyoid  ligaments  being  in  inverse 
ratio  to  the  length  of  the  processes  or  cornua.     The  greater 


TIIK    liKANClllAI,    AKCIl    SKKIKTON. 


205 


rornua  of  the  hyoid  are  formed  by  the  ossification  of  the 
cartilages  of  the  third  arch,  and  the  body  of  the  bone  is 


i  1 


Fig    106 -Diagram  showing  the  Categories  to  which  the  Bones 

OF  THE  Skull  Belong. 
The  unshaded    b..nes  a-   n.njbrane  bone,    the   sl.ade^   ^^S^Mch 

//'  interpariet  il;  M,  malar;  Mv,  mandible;  Mx,  "wxdla ,  A .4 .  nasa  , 
7^  arielal;  Pc  periotic;  50,  supraoccipital ;  .Sg,  «i«amosal .  St. 
st'yloid  process;  Th,  thyreoid  cartilage;  Ty.  tympanic 

formed  from  a  cartilaginous  plate,  the  copula,  which  unites 
the  ventral  ends  of  the  two  arches  concerned. 


2oCt  TIIK    r»F.VI-.I.OfMi:NT    OK     illK    HUMAN    IIODV. 

I'inallv  the  cartilai^ts  of  the  fourth  and  fifth  arehes  early 
fuse  totjetlier  to  form  a  i)hite  of  eartihi>,'e,  and  the  two  plates 
of  opposite  sides  iniite  by  their  ventral  edi^es  to  form  the 
thyreoid  cartilatce  of  the  larynx. 

The  accompanyinj,^  diagram  (I'iij.  io6)  shows  the  vari- 
ous structures  derived  from  the  branchial  arch  skeleton  as 
well  as  some  of  the  other  elements  of  the  skull,  and  a  re- 
sume of  the  fate  of  the  branchial  arches  may  be  stated 
in  tabular  form  as  follows,  the  parts  represented  by  car- 
tilage which  becomes  replaced  by  membrane  bone  being 
printed  in  italics,  while  membrane  bones  which  have  no 
cartilaginous  representatives  arc  enclosed  in  brackets : 

;  (Maxilhi). 
I  (P..latine). 
I'ttryt^oid      internal  pterygoid  jjroccss  of  sphenoid. 
1st  arch,     Malleus. 

(Incus. 
Spheno-iiiandiliular  li^'anicnt. 
Mandible. 
I  Stapes. 
_,        .  'styloid   process  of  the   temporal. 

'    j  Stylo-hyoid  ligament. 

'  r.esscr  cornu  of  hyoid. 

.^d  arch Cireatcr  cornu  of  hyoid. 

4th  and  5tii  arches, .  .     Thyreoid  cartilage  of  larynx. 

The  Development  of  the  Appendicular  Skeleton. — While 
the  axial  skeleton  is  formed  from  the  sclerotomes  of  the 
mesodermic  somites,  the  appendicular  skeleton  is  derived 
from  the  somatic  mesenchyme,  which  is  not  divided  into 
metameres.  This  mesenchyme  forms  the  core  of  the  limb 
bud  and  becomes  converted  into  cartilage,  by  the  ossifica- 
tion of  which  all  the  bones  of  the  limbs,  with  the  possible 
exception  of  the  clavicle,  are  formed. 

Of  the  bones  of  the  pectoral  girdle  the  clavicle  re- 
quires further  study  before  it  can  be  certain  whether  it  is 
to  be  regarded  as  a  pure  cartilage  bone  or  a  combination  of 


rilK    I'KITOKAI.    (;IKI>I.F. 


207 


cartilage  and  nuMuhraiu-  ossifu-ations  '(W'Ktnbaur).     It  is 
the  first  botie  of  the  skeleton  to  ossify,  its  center  appearni« 
rt  about  the  sixth  week  of  (leveloi)inent.     The  tissue  in 
which  the  ossification  forms  hascertain  peculiarcharacters, 
and  it  is  dinricult  to  say  whether  it  is  to  he  regarded  as 
cartilage  which,  on  account  of  the  early  differentiation  o 
the  center,  has  not  yet  become  thoroughly  difTerentiated 
histologically,  or  as  some 
other  form  of  connective 
tissue.      However     that 
may  be,  true  cartilage  de- 
velops on  either  side  of 
the  ossifying  region,  and 
into  tliis  the  ossification 
gradually     extends,      so 
that  at  Icas^  a  portiw.i  of 
the  bone  is  preformed  in 
cartilage. 

The  scapula  is  at  first 
a  single  plate  of  cartilage 
in  which  two  centers  of 
ossification  appear.     One 
of  these  gives  rise  to  the 
body  and  the  spine,  while 
the  other   produces    the 
coracoid     process     (Fig. 
107,  co),    the  rudimentary    representative  ot    the  cora- 
coid bone  which  extends  between  the  scapula  and  ster- 
num in  the  lower  vertebrates.      The  coracoid  does  not 
unite  with  the  body  until  about  the  fifteenth  year,  and 
sccondarv  centers  which  give  rise  to  the  vertebral  edge 
(b)    and^  inferior    angle    of   the  bone    (a)    and    to    the 
acromion  process  (c)  unite  with  the  rest  of  the  bone  at 
about  the  twentieth  year. 


Fic. 


107  —The    Ossification    Cen- 
ters OF  THE  ScAPruA. 
,j,    /),   and   c,    vSeconcUiry    centers    for 
'  the    angle,   vertebral    border,    and 
acromion ;  co,  center  f'  -  '  'h-  coracoid 
process. — {Tcstui.) 


20S 


THE    l>KVF.I.<)PMF.Nr   OF    TIIF:    lll'MAN    IH)|)\. 


TIk'  hunums  and  the  hones  of  the  forearm  are  tyj)ical 
lonj;  hones,  caeh  of  which  develops  from  a  primary  center 
wliich  gives  rise  to  the  shaft  and  has,  in  addition,  two  or 
more  epiphysial  centers.  In  the  humerus  an  epiphysial 
center  appears  for  the  he-.d,  another  for  the  greater  tuber- 
osity, and  usually  a  third  for  the  lesser  tuberosity,  while 
at  the  distal  end  there  is  a  center  for  each  condyle,  one  for 
the  trochlea  and  one  for  the  capitulum,  the  fusion  of  these 
•arious  epif  lyses  with  the  shaft  taking  place  between 
the  seventeenth  and  twentieth  )  v  .irs.  The  radius  and 
ulna  each  possess  a  single  epiphysial  center  for  each  ex- 
tremity in  addition  to  the  primary  center  for  the  shaft, 
and  the  ulna  possesses  also  an  epiphysial  center  for  the 
ulecrancm  process. 

The  embryological  development  of  the  carpus  is  some- 
what complicated.  A  cartilage  is  found  representing  each 
of  the  bones  normally  occurring  in  the  adult  (Fig.  io8),and 
these  arc  arranged  in  two  distinct  rows:  a  proximal  one 
consisting  of  three  elements,  named  from  iheir  relation  to 
the  bones  of  the  forearm,  radiale,  interrtiedium ,  and  ulnarc; 
and  a  distal  one  composed  of  four  elements,  termed  carpa- 
lia.  In  addition,  a  cartilage,  termed  the  pisiform,  is 
found  on  the  ulnar  side  of  the  proximal  row  and  is  gener- 
ally regarded  as  a  sesamoid  cartilage  developed  in  the 
tendon  of  the  flexor  carpi  ulnaris,  and  furthermore  a 
number  of  inconstant  cartilages  have  been  observed  whose 
significance  in  the  majority  of  cases  is  more  or  less  obscure. 
These  accessory  cartilages  cither  disappear  in  later  stages 
of  development  or  fuse  with  neighboring  cartilages,  or,  in 
rare  cases,  ossify  and  form  distinct  elements  of  the  carpus. 
One  of  them,  however,  occurs  so  frequently  as  almost  to 
deserve  classification  as  a  constant  element ;  it  is  known  as 
the  centrale  (Fig.  io8,  c)  and  occupies  a  position  between 
the  cartilages  of  the  proximal  and  distal  rows  and  appar- 


«•"«  «iiHPi;tT:T3r^i.<jtrK  -^r'i''^:*-  lOPi 


THR    SKELETON    OF    THK    ARM    AM.    MANI..  209 

entlv  corresponds  to  a  cartilage  typically  prcscjt  in  loucr 
orn  s  .nd  ossifying  to  form  a  distinct  bone.     l.>  the  h« 
r^ann.s  itsktt  varies,  as  it  ,nay  either  disappear  or 
u  me  w    h  other  cartilages,  that  with  which  .t  nu.st  «s«- 
"1     fu   1  l)ein«  probahlv  the  radiale.     There  ,s  evidence 
t.    "h.  w  tl^t  another  of  the  accessory  c-tda«es  .untes 
u.n  the  ulnar  ele.nent  of  the  distal  row.  representing 
thecarpale  V  typically 
present  in  lower  forms. 
Ivach     of     the    ele- 
ments    corresj)ondinK 
to  an  adult  hone  f)ssi 
lies  fn.in  a  sinjjle  center 
with  the  exception  of 

carpale    TV-"\',    wliich 

has    two     centers,     a 

further    indication    of 

its   composite   charac- 
ter.    The    rela'ion    of 

the  cartilages   to    the 

adult   bones    may    be 

seen    from    the    table 

given  on  page  212. 

With  regard  to  the  .    w,    , 

nietacarpals  and  phalanges,  it  need  merely  be  sta  ed  ha 
each  develops  from  a  single  primary  center  for  the  shatt 
and  one  secondary  epiphysial  center.  The  pnmary 
center  appears  at  about  the  middle  of  the  shaft  except 
in  the  terminal  phalanges,  in  which  it  appears  at  he 
distal  end  of  the  cartilage.  The  epiphyses  for  the 
metacarpals  are  at  the  distal  ends  of  the  bones  except 
in  the  case  of  tlie  metacarpal  of  the  thumb,  which  re- 
sembles   the    phalanges    in   having  its  epiphysis  at  the 

distal  end. 
18 


Fit-..    lOK. 


Ukconstkiction   nv   .\.\   I'.M 
BRVONic  Carpi  s. 
f  Centrali  ;  cu,  cuneiform,  /«,  semilunar; 
'm     osmaRnum;    ^    pisiform;    vr,    sca- 
phoid;  /,    trapezium;   Ir,   trapezoid;   u, 
unciforiri. 


210 


THE    DEVELOPMENT   OF    THE    HUMAN    HODV. 


T,^chtnnomntate  bone  appears  as  a  somewhat  oval  plate 
of  cartilage  whose  long  axis  is  directed  almost  at  rillt 
angles  to  the  vertebral  column  and  which  isT dose  rcfa 
tion  with  the  fourth  and  fifth  sacral  vertebra      As  devd' 
opment  proceeds  a  rotation  of  the  cartil^  JT  1 

bv  a  slifTiif  .chiff  ;„      r        ■  •  <^J"ilage,  accompanied 

by  a  shght  shifting  of  position,  occurs,  so  that  eventuallv 

the  plate  has  its  long  axis 
almost  parallel  with  the 
vertebral  column  and  is 
in  relation  with  the  first 
three   sacrals.      Ossifica- 
tion   appears    at     three 
points  in  each  cartilage, 
one  in  the  upper  part  to 
form  the  ilium  (Fig.  109, 
in  and  two  in  the  lower 
part,  the  anterior  of  these 
K'iving  rise  to  the  pubis 
ip),  while   the   posterior 
produces      the     ischium 
(i^)-       At     birth      these 
three  bones  are  still  sep- 
arated    from     one     an- 
other   by     a     Y-shaped 
piece  of  cartilage  whose 
three  limbs  meet  at  the 

lum    biif  Utnr  o  ,  'bottom  of  the  acetabu 

urn,  but  later  a  secondary  center  appears  in  this  carti 
lage  and  unites  the  three  bones  together      Tl  e  Jnt '  , 
part  of  the  lower  half       each  original  cartilage  plate  do" 
not  undergo  complete  chondrification,  but  remains  mem 
1-nous,  constituting    the    obturato;  mem  "»"  Xh 
closes  the  obturator  foramen. 

In  addition  to  the  Y -^Ii-.tw.,!   ..         1 

Lo  inc   Y  sJiaped  secondary  center,  other 


Fig.  109.-  The  (  )ssikication  Centers 

OF   THE    f)s    IX.NOMI.NATfM. 

«,  b  c,  and  </,  Secondary  centers  f(,r 
the  crest,  anterior  inferior  snine  of 
syniphysis.  and  ischial  tuhen.sitv 
?/,  ihuin;  ,.v,  ischium;  p,  puhis  ' -' 
(  /  cstut. )  ' 


THE    PELVIC    GIRDLE    AND    LOWER    LIMB    SKELETON. 


21  I 


epiphysial  centers  appear  in  the  prominent  portions  of  the 
cartilase.  such  as  the  pubie  crest  (Fig.  109  c).  the  isch.a 
tuberosity  (d),  the  anterior  inferior  spine  (&)  and  the  crest 
of  the  iUum  (a),  and  unite  with  the  rest  of  the  bone  at 
about  the  twentieth  year.  . 

The  femur,  tibia,  and  jihula  each  develop  from  a  smgle 
primary  center  for  the  shaft  and  an  upper  and  a  lower 
epiphysial  center,  the  femur  possessing,  in  addition,  epi- 
physial centers  for  the  greater  and  lesser  trochanters 
( Fig  90)      The  patella  does  not  belong  to  the  same  cate- 
gory as  the  other  bones,  but  resembles  the  pisiform  bone 
of  the  carpus  in  being  a  sesamoid  bone  developed  in  the 
tendon  of  the  quadriceps  extensor  cruris.     Its  cartilage 
does  not  appear  until  the  fourth  month  of  intrauterine 
life  when  most  of  the  primary  centers  for  other  bones  have 
already  appeared,  and  its  ossification  does  not  begin  until 
the  third  vear  after  birth. 

The  tarsus,  like  the  carpus,  consists  of  a  proximal  row 
of  three  cartilages,  termed  the  tihialc,  the  ^ntcrmcd^um, 
and  the  fibularc,  and  of  a  distal  row  of  four  tarsaha.    Be- 
tween these  two  rows  a  single  cartilage,  the  ccntralc,  is 
interposed.     Each  of  these  cartilages  ossifies  from  a  sin- 
crle  center,  that  of  the  intermedium  early  fusing  with  the 
tibiale,  though  it  occasionally  remains  distinct  as  the  os 
trigonum,  and  from  a  comparison  with  lower  forms  it 
seems  probable  that  the  fibular  cartilage  of  the  distal  row 
really  represents   two    separate    elements,   there    being, 
properly  speaking,   five  tarsalia  instead  of  four.     The 
fibularc,  in  addition  to  its  primary  center,  possesses  also 
an  epiphysial  center,  which  develops  at  the  point  of  inser- 
tion of  the  tendo  Achillis. 

A  comparison  of  the  carpal  and  tarsal  cartilages  and 
their  relations  to  the  adult  bones  may  be  seen  from  tlie 
following  table: 


212 


THE    DEVELOPMENT   OF    THE    HUMAN    BODY 


Carpus. 


Takm-^. 


Cartilage!!. 

Radialc 
Internicdiiiiii 
Ulnare 
Sesamoid 

laije 
Cenlrale 


Bones. 


cartl- 


Carpale  I 
Carpale   If 
Carpalc  III 
Carpale  IV 
Carpale  V 


} 


Scaphoid 
Semilunar 
Cuneiform 
Pisiform 

,  Fuses    with    sca- 
phoid 
Tra[)ezium 
Trapezoid 
Os  .Magnum 

Unciform 


i  Bones. 

I 

Astragahis 

Os  Calcis 

Navicular 

Int.  Ctmeiform 
Mid.  Cuneiform 
Hxt.  Cuneiform 

Cuboid  J 


Cartilages. 


Tibiale 

Intermedium 

Fibulare 


Centra  le 

'larsaie  I 
Tarsaie  H 
Tarsaie  III 
I'arsale  I\' 
Tarsaie  V^ 


The  development  of  the  metatarsals  and  phalanges  is 

Tard'trp't;;. '"" "'  "■=  --^"'^"^ '-  ^^ 

The  Development  of  the  Jolnts.-TI,e    mesenchvme 

the  skeleton  of  a  hmb,  ,s  at  first  a  continuous  mass  and 
when  ,t  beeomes  converted  into  cartilage  this  al  o  raav  b 
contmuous.  as  in  the  skull,  or  may  appL  as  a  nunTlSr 

hej  exti"     ;?,  'o™-.--  'he  various  ossifications  as 
7     n  """""^  '"'°  ™"taet  With  their  nciirhbors 

nd  w,  1  ether  fuse  with  them  or  will  articulate  S  h'm 
directly,  formmg  a  sulure. 

When,  however,  a  portion  of  unmodified  mesenchmve 
ntervenes  between  two  cartilages,  the  mode  of  a      X 
t.on  of  the  bones  formed  from  these  cartilages  will  va 
1  h   mtermedmte  mesenchyme  may  in  time  undergo  ch„n ' 

artaulation  known  as  a  sp,cl,o„d>ons  (<■.  „     the  s-,cro 
.  .ue  articulation , :  or  a  cavity  may  appear  in  th  tnt'er    f 
.1.0  mtervenmg  cartilage  so  that  a  slight  amount  of  move 


THK    JOINTS. 


13 


ment  of  the  two  bones  is  possible,  forming  an  amphiarthro- 
sis  (e  g  ,  the  symphysis  pubis) ;  or,  finally ,  the  intermediate 
mesenchyme  may  not  chondrify,  but  its  peripheral  por- 
tions may  become  converted  into  a  dense  sheath  of  con- 
nective tissue  (Fig.  no,  c)  which  surrounds  the  adjacent 
ends  of  the  two  bones  like  a  sleeve,  forming  the  capsular 
ligament,  while  the  central  portions  degenerate  to  form  a 
cavity.  The  bones  which  enter  into  such  an  articulation 
are  more  or  less  freely  movable  upon  one  another  and  the 


c — 


r.<-.    110-LoN.;iTun.N..L  Sectio.v  through  the  Joint  of  the  Great 
Toe  in  .\n  Embryo  of  4.5  cm. 

second  pluiUinges.--  (.\  icolas.) 

joint  is  termed  a  diarthrosis  {c.  g.,  the  knee-  or  shoulder- 
joint). 

In  a  diarthrosis  the  connective-tissue  cells  near  the 
inner  surface  of  the  capsule  arrange  themselves  in  a  layer 
to  for-  a  synovial  membrane  for  the  joint,  and  portions 
of  th  capsule  may  thicken  to  form  special  bands,  the  rein- 
forcing ligaments,  while  other  strong  fibrous  bands,  which 
may  pass  from  one  bone  to  the  other  forming  accessory 
ligaments,  are  shown  by  comparative  studies  to  be  in 


214 


THE    DEVELOPMKNT    OF    THE    HUMAN    HODV, 


many  cases  degenerated  portions  of  what  were  originally 
muscles. 

In  certain  diarthroses,  such  as  the  temporo-mandibular 
and  claviculo-sternal,  the  whole  of  the  central  portions  of 
the  intermediate  mesenchyme  does  not  degenerate,  but 
It  is  converted  into  a  fibro-cartilage,  between  each  surface 
of  which  and  the  adjacent  bone  there  is  a  cavity.  These 
interarticular  cartilages  seem,  in  the  sternoclavicular 
jomts,  to  represent  the  sternal  ends  of  a  bone  existing  in 
lower  vertebrates  and  known  as  the  precoracoid,  biit  it 
seems  doubtful  if  those  of  the  temporo-mandibular  j(  ts 
have  a  similar  significance. 


fM— 


LITERATURE. 

A.  Bkrnavs:  "Die  KntwicklungsKeschiclitc  des  Kniegelenks  des  Menschen 
nut  Bernerkungen  iiber  die  Gelenke  i.„  Allgeineinen."  Morbholo, 
Jahibuch,  iv,  1878.  ' 

E.  Di  Rsv:  "Zur  Kntwieklungsgescliiclue  des  Kopfes  des  Menschen  und 

der  hoheren  Wirbelthiere,"  Tubingen,  1869 
V.  vox  Ebner:  "Ueber  die  Beziehungen  der  Wirbel  zu  den  Unvirbeln  ■' 

Sitzungsbenchte  der  kais.   Akad.   Wnn,  ci,  3te  Abth      1892 
A.  Pror.ep:  "Zur  Entwicklungsgesohichte  der  Wirbelsaule,  insbesondere 
des  Atlas  und    Epistropheus  und   der  Occii.italregi-.n  "    \rchiv  jur 
Anat.  und  I'hysioi,  Anat.   Abth.,    1886. 
C.   GEr.E.VB.UK:  "Kin   Kail   vcn  erblichem   Mangel  der   Pars   acro.nialis 
Uaviculx,  nut  Benierkungen  ul,er  die  Kntvvicklung  der  Chivicula  " 
Jetiaisclit  Zntschr.  ju,    nudtr.   UisscHsch  ,  i     1864 
Henkk  .xxd  Revher:  "Studien  iiber  die  Entwickelung  der  Extremitaten 
des  Mensehen,  msbes.ndere  der  Gelenktlachen,"  Sif,u„>i.d>cnchtc  d,r 
kais.  Akad.   Wici,  Lxx,    1875. 
M.  J.XKOBV:  "  Beitrag  zur  Kenntnisdes  menschlichen  rri.n.,r<iialcraniunis  - 

Archil'  jur  mikrosk.  Aunt.,  xi.iv,    1894 
H.   I.EBOrco:  "  Kecherches  sur  la  .n;,rph,.li,j,ne  du   carpe   ehez  les  n.a.n- 

nuferes,      Archives-  d,    lUolog.,   v,    1884. 
(i.  Levi:  "Beitrag  zum  Studium  der  Entvviekelung  des  kn..rpe]igen   I'ri- 
morchalcramums  des  Mensehen,"  Archir  ju,  niikro.sk.  .-Uuit..  u-,  1900 
1.1.  M.w.u:    •  The  Development  of  the  Connective  tissues  f -om  the  C.n- 

nective-tissue  Syncytium,"  Amcr.  Jour.  Anat     i    1902 
W.  VAX  \ooR„Ex:  "Beitrag  zur  Anatomic  der  km.rpeligen  Schadelbasis 


I.ITKKATUKE. 


5 


nienschlicher    K.nbryonen."    Arclnv   }ur   Anat    nud   Pkysu,!..    -UnU 

R^MBu'T'Et'REN^ruT:  "Online  et  develuppen.ent  des  Os."  Paris,  1S64. 
lZ^^^V.^.r  die  .itvvickclnn,  der  Wirhelsaule  und  das  C.ntrale 

carpi  des  Menschen,"  Morpholoi^.  !uhrhucl,.  i,  18,6. 
G    Riu'e-  "UntersuchunKcn  iiber  die  KnUvickelungsv..rgan«e  an,  hr«.,t- 

hein  des  Menschen,"  Morphoh^.  Jnhrhuch,  vi,  1880. 
T     "emus:    -Untersuchungen   uher   die   morplu.logische    Becleu  ung 

a"cessorischer    F.le.nente   am   menschlichen   Carpus   (und   larsus), 

Morbholoe..   Arbcihn,   v,    1896. 
P    A.   Z.;cH.XKi.xn^s:  "Recherches  sur  le  d-elopi^ement  du  t.ssu  cou- 

junctiv,"  Comptis  Rendu,  dc  la  Soc  de  lUolog.  Parts,  ber  10.  v,  18)8. 


c,. 


CHAPTKR   VIII. 

THE  DEVELOPMENT  OF  THE  MUSCULAR 

SYSTEM. 

Two  forms  of  muscular  tissue  exist  in  the  liumun  body 
the  striated  tissue,  which  forms  the  skeletal  muscles  and 'is 
under  the  control  of  the  central  nervous  system,  and  the 
non-striated,    which   is   controlled   by   the   sympathetic 
nervous  system  and  is  found  in  the  skin,  in  the  walls  of  the 
digestive  tract,  the  blood-vessels  and  Ivmphatics,  and  in 
connection  with  the  genito-urinary  apparatus.     In  the 
walls  of  the  heart  a  muscle  tissue  occurs  which  is  frequently 
regarded  as  a  third  form,  characterized  by  being  under 
control  of  the  sympathetic  system  and  vet  being  striated  • 
It  is,  however,  in  its  origin,  much  more  nearly  allied  to  the 
non-striated  than  to  the  striated  form  of  tissue,  and  will 
be  considered  a  variety  of  the  former. 

The  Histogenesis  of  Non=striated  Muscular  Tissue.- 
Non-striated  muscular  tissue  is  formed  bv  the  direct  con- 
version of  mesenchyme  cells  into  muscle-fibers,  the  exact 
details  of  the  conversion  being  as  yet  unknown.  The 
fibers  are  sometinics  more  or  less  scattered  in  the  general 
connective  tissue  or  may  be  grouped  into  small  bundles 
or  into  layers.  They  are  formed  from  the  mesenchyme 
of  the  dermatomes  and  from  that  of  the  somatic  and 
splanchnic  layers  of  the  mesoderm,  but  never  from  the 
myotomes  of  the  mesodermic  somites. 

The  cells  from  which  the  heart  musculature  develops 
sliow  at  first  an  irregular  protoplasmic  reticulum  (Fig 
1 1 1,  .-1 )  which  later  becomes  regularly  arranged  so  as  to 

216 


2SSSSS 


mm 


THK    DKVKI.OPMKNT    OF    MUSCI.K    TISSUK. 


21 


give  the  cell  when  viewed  in  longitudinal  section  the  ap- 
pearance of  being  composed  of  a  series  of  disks  arranged  in 
elosely  approximated  rows,  each  disk  being  one  of  the 
meshes  bounded  bv  the  reticulum   fibers.     Later  each 
mesh  or  disk  (Fig.  in,  B)  becomes  divided  into  smaller 
disks  by  reticulum  trabeculae  which  meet  in  the  centers  of 
the  original  disks,  and  at  the  lines  along  which  these  sec- 
ondary trabecule  meet  the  reticulum  thickens  to  form  a 
fibril  (Fig.  1 1 1 ,  C,  /).      The  formation  of  the  fibrils  begms 
at  the  periphery  of  the  cell  and  proceeds  centrally,  though 
even  in  the  adult  condition  there  is  an  area  surroundmg 


Vic.    111. -Cross-sections      f    He.vrt-miscle   Cells    from   Pio   Em- 
bryos OF  (.A)    10  MM.   AND  (li  AND   L)   20  MM. 

/,  Fibril;  /,  large  disk;  «,  nucleus;  s,  small  disk. -{Maailhim.) 

the  nucleus  in  which  they  do  not  develop.  The  cells  so 
altered  arrange  themselves  at  first  in  bundles  distinctly 
separated  from  one  another,  so  that  the  heart-wall  has 
a  somewhat  spongy  appearance,  but  later  the  various 
bundles  fuse  more  or  less  completely  to  form  a  solid  mass, 
the  original  conditioi  ing  retained  only  in  the  auricular 
appendices  and  on  tiie  inner  surfaces  of  the  ventricles, 
where  the  bundles  form  the  columnae  earner  and  musculi 

papillares. 

The  Histogenesis  of  Striated  Muscular  Tissue.— The  his- 
togenesis of  the  striated   muscle-fibers  resembles  very 


2l8 


THE    DEVELOPMENT    OF    THE    HUMAN    BODY. 


i 


closely  that  described  as  occurring  in  the  Iieart  n^uscle, 
with  the  difference  that  the  fibrils  are  developed  through- 
out the  entire  thickness  of  the  cell,  the  nucleus  originally 
present  disappearing,  while  new  nuclei  (Fig.  112,  B),  in 
considerable  number,  make  their  appearance  at  the  per- 
iphery of  the  fiber,  some  of  these  being  possibly  formed  by 
a  division  of  the  original  nucleus.  The  formation  of  the 
fibrils  is  completed  in  embryos  of  about  17  cm.  in  lengtli, 
and  up  to  this  period  the  increase  in  thickness  of  a  muscle 


Fig.  112. — Cross-section  of  a  Muscle  from  the  Thk.ii  of  .\  Pic  Hm- 

BRYO   75   .MM.    Lo.NC. 

.4,   Original  central  nucleus;  B,  new  peripheral    nucleus.     (.)/<if(j//M»j.) 

is  probably  due  to  a  certain  extent  to  an  increase  in  the 
actual  number  of  fibers,  ne'.v  fibers  forming  by  the  division 
of  those  already  existing.  Subsequently,  however,  this 
mode  of  growth  ceases,  the  further  increase  of  the  muscle 
depending  upon  an  increase  in  size  of  its  constituent  ele- 
ments (Macullum). 

The   Development   of  the   Skeletal    Muscles. — It   has 
already  been  pointed  out  that  all  the  skeletal  muscles  of 


Tin:   SKKI.l  lAI.    MUSCI.KS. 


219 


the  body,  with  the  exception  of  those  connected  with  he 
branchial  arches,  are  derived  from  the  myotomes  of  the 
mesodermic  somites,  even  the  Hmb  muscles  probably  luu  - 
ing  such  an  origin,  although  the  cells  of  the  myotomes  as 
they  grow  out  into  the  limb  buds  early  lose  their  epithe- 
lial arrangement  and  become  indistinguislmble  from  he 
somatic  mesenchyn..  which  forms  the  axial  core  of  the 

'"tI^c  various  fibrils  of  each  myotome  are  at  first  loosely 
arranged,  but  later  tl.ey  become  more  compact  and  are 
arranged  parallel  with  one  another,  their  long  axes  being 
directed  antero-posteri.  .rlv.     This  stage  is  also  transitory , 
however,  the  fibers  of  each  myotome  undergoing  various 
modifications  to  produce  the  conditions  existmg  in  the 
adult,  in  which  the  original  segmental  arrangement  of  the 
fibers  can  be  perceived  in  comparatively  few  muscles     Tlie 
exact  nature  of  these  modifications  is  almost  unknown 
from  direct  observation,  but  since  the  relation  between  a 
nerve  and  the  myotome  belonging  to  the  same  segment  is 
established  at  a  very  early  period  of  development  and  per- 
sists throughout  life,  no  matter  what  changes  of  fusion, 
splitting,  or  migration  the  myotome  may  undergo,  it  is 
possible  to  trace  out  more  or  less  completely  the  history  o 
the  various  myotomes  by  determining  their  segmental 
innervation.     It  is  known,  for  example,  that  the  latissi- 
mus  dorsi  arises  from  the  seventh  and  eighth*  cervical 
myotomes,  but  later  undergoes  a  migration,  becoming  at- 
tached to  the  lower  thoracic  and  lumbar  vertebrae  and  to 
the  crest  of  the  iHum,  far  away  from  its  place  of  origin 
(Mall),  and  yet  it  retains  its  nerve-supply  from  the  seventh 
and  eighth  cervical  nerves  with  which  it  was  originally 

'  *  This  enun.eration  is  based  on  convenience  in  associating  t!-"  '»yo- 
tomeswith  tlie  nerves  ^^•lnch  supply  them.  The  myoto.nes  ment.oned 
are    ho"e  which  correspon.l  to  the  s.xth  and  seventh  cerv.cal  vertehrx. 


!20 


THE    nFA'EI.OPMENT    OF    THE    HUMAN    BODV. 


f<    I 


associated,  its  nerve-supply  consequently  indicating  the 
extent  of  its  migration. 

By  following  the  indications  thus  afforded,  it  may  be 
seen  that  the  changes  which  occur  in  the  myotomes  may 
be  referred  to  one  or  more  of  the  following  processes : 

1.  A  longitudinal  splitting  into  two  or  more  portions, 
a  process  well  illustrated  by  the  trapezius  and  sterno- 
mastoid,  which  have  differentiated  by  the  longitudinal 
splitting  of  a  single  sheet  and  contain  therefore  portions 
of  the  same  myotomes.  The  sternohyoid  and  omohyoid 
have  also  differentiated  by  the  same  process,  and,  indeed, 
it  is  of  frequent  occurrence. 

2.  A  tangential  splitting  into  two  or  more  layers.  ICx- 
amples  of  this  are  also  abundant  and  are  afforded  by  the 
muscles  of  the  fourth,  fifth,  and  sixth  layers  of  the  back, 
as  recognized  in  English  text -books  of  anatomy,  by  the 
two  oblique  and  the  transverse  layers  of  the  abdominal 
walls,  and  by  the  intercostal  muscles  and  the  triangularis 
sterni  of  the  thorax. 

3.  A  fusion  of  portions  of  successive  myotomes  to  form 
a  single  muscle,  again  a  process  of  frequent  occurrence, 
and  well  illustrated  by  the  rectus  abdominis  (which  is 
formed  by  the  fusion  of  the  ventral  portions  of  the  last  six 
or  seven  tlioracic  myotonies)  or  by  the  superficial  portions 
of  the  erector  spinae. 

4.  A  migration  of  parts  of  one  or  more  myotomes 
over  others.  An  example  of  this  process  is  to  be  found  in 
the  latissimus  dorsi,  whose  history  has  already  been  re- 
ferred to,  and  it  is  also  beautifully  shown  by  the  serratus 
magnus  and  the  trapezius,  both  of  wliich  have  extended 
far  beyond  the  limits  of  the  segments  from  which  they  are 
derived. 

5.  A  degeneration  of  portions  or  the  whole  of  a  myo- 
tome.    This  process  has  played  a  very  considerable  part 


»*lwan 


THE    SKELETAL    MUSCLES. 

in  the  evolution  of  the  muscular  system  in  the  vertebrates. 
When  a  muscle  normallydeKcnerates,  it  becomes  co.i- 
verted  into  connective  tissue,  and  many  of  the  strong 
aponeurotic  sheets  which  occur  in  the  body  owe  the.r 
origin  to  this  process.     Thus,  for  example,  the  aponeurosis 
connecting  the  occipital  and  frontal  portions  o    the  occ- 
pito  frontalis  is  due  to  this  process  and  is  muscular  m  such 
forms  as  the  lower  monkeys,  and  a  good  example  is  alsc, 
to  be  found  in  the  aponeurosis  which  occupies  the  interval 
between  the  superior  and  inferior  serrati  post ici.  these  two 
.nuscles  being  continuous  in  lower  forms.     1  he  strong 
lumbar  aponeurosis  and  the  aponeuroses  of  the  oblique 
and  transverse  muscles  of  the  abdomen  are  also  good  ex- 

""Tideed,  in  comparing  one  of  the  mammals  with  a  mem- 
ber of  one  of  the  lower  classes  of  vertebrates,  the  greater 
amount  of  conr  "tivc  tissue  compared  with  the  amount 
of  muscular  tissue  m  the  former  is  very  striking,  the  infer- 
ence being  that  these  connective-tissue  structures  (fascia>, 
aponeuroses,  ligaments)  represent  portions  of  the  muscu- 
llr  tissue  of  the  lower  form  (Barddeben).     Many  of  the 
accessorv  ligaments  occurring  in  connection  with  diar- 
throdiarjoints  apparently  owe  their  origin  to  a  degenera- 
tion of  muscle  tissue,  the  external  lateral  ligament  of  the 
knee-joint,  for  inst.    .c,  being  probably  a  degenerated 
portion  of  the  peroneus  longus,  while  the  great  sacro- 
sciatic  ligament  appears  to  stand  in  a  similar  relation  to 
the  long  head  of  the  biceps  femoris  (Sutton). 

6  Finally,  there  mav  be  associated  with  any  of  the  first 
four  processes  a  change  in  the  direction  of  the  musx^le- 
fibers  The  original  anteroposterior  direction  of  the  fibers 
is  retained  in  comparatively  few  of  the  adult  muscles  and 
excellent  examples  of  the  process  here  referred  to  are  to  be 
found  in  the  intercostal  muscles  and  the  muscles  of  the 


»=:; 

Q 


222 


THK    PKVEI.OPMF.NT    OF    TIIK    Ul'MAN    HOPV. 


abdominal  walls.  In  the  musculature  associated  with  the 
htanchial  arches  the  alteration  in  the  direction  of  the  fibers 
occurs  even  in  the  fishes,  in  which  the  original  ilirection  of 
the  muscle  fibers  is  very  jx'rfectly  retained  in  other  myo- 
tomes, the  branchial  muscles,  however,  being  arranged 
parallel  with  the  branchial  cartilages  or  even  passing 
dorso-ventrally  between  the  upper  and  lower  portions  of 
an  arch,  and  so  forming  what  may  be  regarded  as  a  con- 
strictor of  the  arch.  This  alteration  of  direction  dates 
back  so  far  that  the  constrictor  arrangement  may  well  be 
taken  as  the  primary  condition  in  studying  the  changes 
whicli  the  branchial  musculature  has  undergone  in  the 
mammalia. 

It  would  occupy  too  nuicli  pace  in  a  work  of  this  kind 
to  consider  in  detail  the  history  of  each  skeletal  nuiscle  of 
the  human  body,  but  a  statement  of  the  general  plan  of 
their  development  will  not  be  out  of  place.  I-'or  conve- 
nience the  entire  system  may  be  divided  into  three  por- 
tions— the  cranial,  trunk  and  limb  nmsculature;  and  of 
these,  the  trunk  musculature  mav  first  be  considered. 

The  Trunk  Musculature. — It  has  already  been  seen  fp. 
124)  that  the  myotomes  at  first  occupy  a  dorsal  position, 
becoming  prolonged  ventrally  as  development  proceeds 
so  as  to  overlap  the  somatic  mesoderm,  until  those  of 
opposite  sides  come  into  contact  in  the  mid-ventral  line. 
Before  this  is  accomplished,  however,  a  longitudinal 
splitting  of  each  myotome  occurs,  whereby  there  is  sepa- 
rated off  a  dorsal  portion  which  gives  rise  to  a  segment  of 
the  dorsal  musculature  of  the  trunk  and  is  supplied  by  the 
ramus  dorsalis  of  its  corresponding  spinal  nerve.  In  the 
lower  vertebrates  this  separation  of  each  of  the  trunk 
myotomes  into  a  dorsal  and  ventral  portion  is  much  more 
distinct  in  the  adult  than  it  is  in  man,  the  two  portions 
being  separated  by  a  horizontal  plate  of  connective  tissue 


:-"""- -     ^«l 


TIIK   TRUNK    Ml'SCl'I.ATURE. 


223 


extending  the  entire  length  of  the  trunk  and  being  at- 
tached by  its  inner  edge  to  the  transverse  processes  of  the 
vertebra",  while  peripherally  it  becomes  continuous  with 
the  connective  tissue  of  the  dermis  along  a  line  known  as 
the  lateral  line.  In  man  the  dorsal  portion  is  also  much 
smaller  in  proportion  to  the  ventral  portion  than  in  the 
lower  vertebrates.     I'rom  this  dorsal  portion  of  the  myo- 


Fic    in  -Embryo  of  13  mm.  showinc  the  roKMATio.N  of  thb  Rectus 

tomes  arc  derived  the  muscles  belonging  to  the  three 
deepest  layers  of  the  dorsal  musculature,  the  more  super- 
ficial layers  being  composed  of  muscles  belonging  to  the 
limb  system.  Further  longitudinal  and  tangential  divi- 
sions and  a  fusion  of  successive  myotomes  bring  about  the 
conditions  which  obtain  in  the  adult  dorsal  musculature. 


THE    DKVELOrMENT    OF   THE    HUMAN    BODY. 


I 
I 


1 


While  the  myotomes  arc  still  some  distance  from  the 
mid-ventral  line  another  longitudinal  division  adects 
their  ventral  edges  (Fig.  113),  portions  being  thus  sepa- 
rated which  later  fuse  more  or  less  perfectly  to  form  longi- 
tudinal iKinds  of  muscle,  those  of  opposite  sides  being 
brought  into  apposition  along  the  mid-ventral  line  by  the 
continued  growth  ventrally  of  the  myotomes.  In  this 
way  are  formed  the  rectus  and  pyramidalis  muscles  of  the 
abdomen  and  the  depressors  of  the  hyoid  bone,  the  genio- 
hyoid and  genio-hyo-glossus  *  in  the  neck  region.  In  the 
thoracic  region  this  rectus  set  of  muscles,  as  it  may  be 
termed,  is  not  represented  except  as  an  anomaly,  its  al)- 
sence  being  probably  correlated  with  the  development  of 
the  sternum  in  this  region. 

The  lateral  portions  of  the  myotomes  which  intervene 
between  the  dorsal  and  rectus  muscles  divide  tangentially, 
producing  from  their  dorsal  portions  in  the  cervical  and 
lumbar  regions  muscles,  such  as  the  longus  colli  and  psoas, 
which  lie  beneath  the  vertebral  column  and  hence  have 
been  termed  hyposkeletal  muscles  (Huxley).     More  ven- 
trally three  sheets  of  muscles,  lying  one  above  the  other,  are 
formed,  the  libers  of  each  sheet  being  arranged  in  a  definite 
direction  differing  from  that  found  in  the  other  sheets. 
In  the  abdomen  there  are  thus  formed  the  two  oblique  and 
the  transversalis  muscles,  in  the  thorax  the  intercostals 
and  the  triangularis  sterni,  while  in  the  neck  these  portions 
of  some  of  the  myotomes  disappear,  those  of  the  remainder 
giving  rise  to  the  scaleni  muscles,  portions  of  the  trapezius 
and  sternomastoid  (Bolk),  and  possibly  the   hyoglossus 
and  styloglossus.      In  the  abdominal  region,  and  to  a  con- 
siderable extent  in  the  neck  also,  the  various  portions  of 


*  Tills  ■iiiisclf  's  snpiilii'd  l)y  the  liyi)(>K'l<'Ss:>l  nervi',  l)Ut  for  tlif  prest-nt 
purposi-  it  i-  c.iivciiicm  to  rt'Kanl  tins  as  a  s])inal  tu-rvc,  as  indeed  it 
|)riiiiarily  is. 


THK   TRUNK    MUSCULATURE. 


225 


"\te?ahk.  on  page  ..6  will  show  the  relation  of  the 
va^u  AtnU  ,nuU.  to  the  portions  of  t;.-y"'o-s 

The  intimate  association  hetwecn  the  peh  ic  gnUlc  am 
the  axia"  skeleton  brings  about  extensive  mochheattons  of 
1  e  posterior  trunk  myoton.es.    So  far  as  the.r  dorsal  po  - 
iiiL  posit-ii^j  J  these  nivotoines  as  tar 

tions  are  concerned  probably  all  these  m^ 
back  as  the  fifth  sacral  arc  represented  in  the  erector 
sphKC  but  the  ventral  portions  frotn  the  first  lumbar  myo- 
T       nm;,r  Is  are  greatly  modified.     The  last  myotome 
t:krn;"rtn«:e  formation  of  the  rectus  abdominis  ,s 
t  twe  fth  thoracic  and  the  last  to  be  «P/-™  «  ;"     ; 
iteral  musculature  of  the  abdomen  .s  the  A-^   ^'^   • 
the  ventral  portions  of  the  remam.ng  lumbar  and  of    1  c 
first  and  second  sacral  myotomes  bemg  devoted  to  the 
formation  of  the  musculature  of  the  lower  limb. 

The  ventral  portionsof  thethird  and  fourth  sacral  myo- 
tomes are  represented,  however,  by  the  levator  am  an<l 
o  c;^eus,  and  are  the  last  myotomes  which  persist  as  mus- 
c^e?  n  the  human  body,  although  traces  of  ^^^^^ 
terior  myotomes  are  to  be  found  in  muscles  such  as  th 
lurvatorcoccygis  sometimes  developed  in  connection  with 

tlip  coccveeal  vertebrae. 

The  perineal  muscles  and  the  external  splnncter  an.  are 

also  developments  of  the  third  and  fourth  (and  second 
acral  myotomes.     At  a  time  when  the  cloaca  (see  p.  .9 

is  still  present,  a  sheet  of  muscles  lymg  close  beneath  the 

integument  forms  a  sphincter  around  its  opemng  Fig.  1 14). 
On  Uie  development  of  the  partition  vvhiC  divides  the 
cloaca  into  rectal  and  urinogenital  portions,  the  sphincter 
is  also  divided,  its  more  posterior  portion  persistmg  as  the 
external  .sphincter  ani,  while  the  anterior  part  gradually 


\ 


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226 


THE    DEVELOPMENT    OF    THE    HUMAN    BOUY, 


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TllK    CRANIAL    MUSCULATURE. 


227 


differentiates  into  the  various  perineal  muscles  (Popow- 

'"the  Cranial  Musculature. -As  was  pointed  out  in  an 
earlier  ctapter,  the  existence  of  distinct  mesodernnc 
^es  ha^  not  yet  been  completely  '^'^^^^'^ 
the  head  of  the  human  embryo,  but  m  lower  forms,  such  as 
tie  eLsmobranch  fishes,  they  are  clearly  <l,stn,gu,shabe^ 
and  U  may  be  supposed  that  their  indistinctness  m  man 
•s  a  scWary  condition.  Exactly  how  many  o  these 
somites  are  represented  in  n,e  nrammalian  head  ,t  ,s  nn- 


oossible  to  say,  but  it  seems  probable,  from  comparison 
!vU  lower  forms,  that  there  a  considerable  nun.bcr. 
T  c  maTority  of  them,  however,  early  undergo  degenera- 
tion "Jin  the  adult  condition  only  three  are  recogn.z- 
able'  two  of  which  are  pr^oral  in  position  and  one  post- 
o*:  The  myotomes  of  the  anterior  pr«,ral  segm  nt 
give  rise  to  the  muscles  of  the  eye  supp bed  by  t le  tlnr 
cranial  nerve,  those  of  the  posterior  one  furnish  the  supe 
orTblique  muscles  imtervated  by  the  fourth  nerve,  wh.le 


i 


!8 


THE    OEVEI.OPMKNT    OI'    THE    HUMAN    BODY. 


from  the  postoral  myotomes  the  external  recti,  supplied 
by  the  sixth  nerve,  are  developed.  The  muscles  supplied 
by  the  hypoglossal  nerve  are  also  derived  from  myotomes, 
but  they  have  already  been  considered  in  connection  with 
the  trunk  musculature. 

The  emaining  nmscles  of  the  head  dilTer  from  all  other 
voluntary  muscles  of  the  body  in  the  fact  that  they  arc 
derived  from  th.e  branchiomeres  formed  by  the  sej^menta- 
tion  of  the  cephalic  ventral  i.icsoderm.  These  muscles, 
therefore,  are  not  to  be  regarded  as  equivalent  to  the  myo- 
tomic  muscles  if  their  embryological  origin  is  to  be  taken 
as  a  criterion  of  equivalency,  and  in  their  case  it  would 
seem,  from  the  fact  that  they  are  innervated  by  nerves 
fundamentally  distinct  from  those  which  supply  the 
myotomic  muscles,  that  this  criterion  is  a  good  one.  They 
must  be  regarded,  therefore,  as  belonging  to  a  special 
category,  and  may  be  termed  hranchiovicric  nmscles  to 
distinguish  them  from  the  myotomic  set. 

If  their  enibr>ological  origin  be  taken  as  a  basis  for  homology, 
it  is  clear  that  they  should  be  regarded  as  equivalent  to  the 
muscles  derived  from  the  ventral  mesoderm  of  the  trunk,  and 
these,  as  has  been  seen,  are  the  non-striated  muscles  associated 
with  the  viscera,  among  which  may  be  included  the  striated 
heart  muscle.  At  first  sight  this  homology'  seems  decidedly 
strained,  chielly  because  long-continued  custom  has  regarded 
the  histological  and  physiological  peculiarities  of  striated  and 
non-striated  muscle  tissue  as  fundamental.  It  may  be  pointed 
out,  however,  that  the  branchiomeric  muscles  are,  strictly  speak- 
mg,  visceral  muscles,  and  indeed  give  rise  to  muscle  sheets 
(the  constrictors  of  the  pharjux)  which  surround  the  upper 
or  pharyngeal  portion  of  the  digestive  tract.  It  is  possible, 
then,  that  the  homologv  is  not  so  strained  as  might  appear, 
but  further  discussion  of  it  may  profitably  be  deferred  until 
the  cranial  nerves  are  under  consideration. 

The  skeleton  of  the  first  branchial  arch  becomes  con- 
verted partly  into  the  jaw  apparatus  and  partly  into  audi- 
tory ossicles, and  the  muscles  derived  from  the  eorrespond- 


'i 


I 


TIIF.    CRANIAL    MUSCULATURE. 


T>( 


r  '*;'"';   rrcl,thetrr«  minus  or   tUth,  ..a  consc- 
branchial    arcli    is   tii«-    «^"h  i-,.,i  v^^- U 

.,„.n„y  those  various  .uusc-les  arc  suppl.  ^  b>     • 
The  seeoml  arch  has  corresponchnR  to  .1  the 
n,..c,  an.  Us  n.useu.a.urc  ^^^^^^l^^^^^^  !:^ 
stylohyohl  and  postcr.or  belly  of  the  uiga 

aovvnwar,!  to  form  a  th.n  "-""f   °     ^^^  ,"  pLtysma 

^""  r jrtin^cCTsct  "hircx.:^^^^  --ar. 

ZXt:^  C'der  of  this  ntuscl.  wjuu.  Us  upp^^ 
parts  beconte  '>>«  X'!™  "  o'«;rcsstn),  t'ogethcr 
•""r':;e  fa  :L  wlia  uni  e  ...e  var  Jus  tnuscles  of  this 
„,th  '1'^  f;^^^'"  ^^^i„„  „f  ,„e  platystna  sheet  of  muscles 
"■"er'thcTacc  is  weTshown  hy  the  dcvdopment  of  the 

?        ,, If  the  facial  nerve  which  supply  U  (F.g.  ns)- 
branches  ot  tnc  laciai  uei  v  tViird  arch 

The  defeneration  of  the  upper  part  of  the  third  arc 

.i^nihcance  and  -^  »  -  ,°  ^'  ^of  the  pharynx, 
r,tS  oTtUrlralso  l.  regarded  as  having  arisen 
'"trtt^eroTSnh  and  hfth  arches  enter  into 

Jtriln-of  the  laryn.  -^^'-Xtu^sr  oTX 
sponding  branchiomcres  constttute  tin   musu 


f. 


Fig.  115.— Head  of  Embryos  (.1)  of  Two  Months  .\nd  (/?)  of  ThreG 
Months  showing  the  Extension  of  the  Seventh  Nerve  upon 
THE  Face. — {Popowsky.) 

230 


THE   LIMll    MUSCl.KS. 


23> 

of  the  con- 


larvnx,  togelhcr  with  the  remaining  porliot 
iuLo.  of\hc  pharynx  and  the  muscR^oUhe.^^^^^ 

;;r:hi.t:"sui:;\ie.t,.en,.^^^^^^^^^^^^^ 

btanchiotnere.  although  the  rcn.aining  muscles  of  tins 

be  i^cJleS  in  the  list  of  branchiomer.c  ■"«--•;";;, 
nuscles  being  the  trapezius  and  ^'""°— ^„    ^  l^j 
already  been  seen  that  these  museks  are  ?="■">  ''^"\ 
t  om  the  cervieal  myotomes,  but  they  also  appear  to  be 

;:rr:ated  m  part  by  the  ^-^^^1^;^:::;. 

«^?;tt:i.tt  pate  :""ows  the  relations  of  the  various 

erJnill  muscles'to'the  myotomes  and  brancnomercs,  as 

^Krp]\  as  to  the  motor  cranial  nerves. 

The  Limb  K«.scles.-In  the  human  etnbryo  the  t.ssue 

fror^  wM,  the  limb  muscles  develop  is  indistmgu.shable 
n  Tarty  stages  from  the  core  of  somatic  mesenchyn.e 
whiel  gives  rise  to  the  limb  skeleton.  And  wh.le  ,1  ^ 
possible' that  the  muscles  may  have  a  commo,,  ongm  w 
the  skeletal  tissue,  yet  it  seems  more  P-'^^^l^^'J^l 
are  really  derived  from  the  myotomes,  and  that  he  unstg 
mented  and  mesenchyu>atous  character  of  *  e  ttssue  ron 
which  they  dilTerentiate  is  a  secondary  condit.on.     1  or 


^^ 


232 


THE    DEVELOPMENT    OF    THE    HUMAN    BOOV. 


I'  ! 


B 
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fe  S  s  5« 

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^3  xJu  C      S 


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-    u    -    u    3 

•r  £  c  .=  — 
i<  ^  ^  ^  — 

A     i^    w    i*  ^ 
"*"*-    *J  >•-     -. 

3  c   C   C   - 
V3  I-.  —  — 


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2  §  5 

^__  rt  ;.; 


^ 


THE    LIMB    MUSCT.F.S. 


233 


seems  certain  that  a  very  considerable  amount  of  con- 
densation of  development  occurs  in  the  limb  muscles; 
prolongations  from  the  myotomes  have  been  observed 
extending  out  into  the  limb  l)uds  in  some  of  the  lower 
vertebrates;  and,  furthermore,  the  distribution  of  the 
nerves  in  the  limbs  of  the  adult  seem  to  indicate  clearly 
a  segmental  arrangement  of  both  the  muscles  and  the 

cutis. 

Accepting,  then,  the  idea  that  the  limb  muscles  are  de- 
rived from  myotomes,  it  may  be  supposed  that  the  myo- 
tomes of  the  segments  corresponding  to  each  limb,  in  their 
growth  ventrally,  extend  outward  over  the  tip  of  the  core 
of  skeletal  mesenchyme  and  return  to  the  side  of  the  trunk 
in  the  manner  shown  in  Fig.  116.     Ivach  myotome  thus 
gives  rise  to  a  portion  of  both  the  dorsal  and  the  ventral 
musculature  of  the  limb  and  forms  a  loop,  as  it  were,  ex- 
tending lengthwise  over  the  axis  of  the  limb.     Since  the 
first  of  the  myotome  loops  which  pass  out  into  each  limb 
lies  along  the  anterior  edge  of  the  limb  bud,  the  muscu- 
lature derived  from  it  will,  in  the  adult,  be  situated  along 
the  outer  side  of  the  arm  and  the  inner  side  of  the  leg, 
because  of  the  opposite  rotation  which  the  two  limbs 
undergo  during  development  (see  p.  107). 

If,  now,  this  loop  idea  be  tested  by  the  distribution  of 
the  nerves  to  the  lower  limb,  it  will  be  found  that  the  first 
myotome  to  pass  out  upon  the  dorsum  of  the  ilium  is  the 
second  lumbar,  and  following  that  there  are  met  succes- 
sively, from  before  backward,  the  remaining  lumbar  and 
the  first  and  second  sacral  myotomes.  The  arrangement 
of  these  myotomes  upon  the  dorsal  surface  of  the  pelvis 
and  the  muscles  to  which  they  contribute  may  be  seen 
from  Fig.  1 17.  In  this  portion  of  their  course  they  repre- 
sent portions  of  the  dorsal  half  of  the  loops,  the  remaining 

portionsextending  downward  on  the  anterior  surface  of  the 


20 


234 


TlIK    DEVKLOI'MKNT    OK     IIIE    HUMAN    ISODY, 


le)ij.  Only  the  sacral  inyotoines,  liowt-vtr,  extt'iid  through- 
out the  entire  lenj^th  of  the  limb  into  the  foot,  the  second 
lumbar  myotc-.e  extending;  dow  n  only  to  about  the  middle 
of  the  thigh,  the  third  to  about  the  knee,  the  fourth  to 
about  the  middle  t)f  the  tibial  region,  and  the  fifth  as  far 
as  the  base  of  the  fifth  metata;     '   bone.      Ivacli  of  these 


FlC.    IK).  —  DlAiiKAM  OF   A   SKUMKNT  OI'   THE    MoDY  AM)    I.IMU. 

bl,  Axial  l)lastein;i;  ilm,  dorsal  musculature  of  body;  il,  nerve  to  linili; 
s,  septum  i)etvveen  dorsal  and  ventral  nnisctdaturc;  .\tr.it,  dorsal 
layer  of  limb  musculature;  ti.il  and  tr.v,  dorsal  and  vet.lral  divisions 
of  a  spinal  nerve;  im,  ventral  musculature  of  the  l)ody, — (Kollnhinn.) 


myotomes  at  the  point  indicated  bends  toward  the  inner 
side  of  the  leg  and  passes  upward  again  on  its  posterior  sur- 
face toward  the  trunk,  representing  in  this  portion  of  its 
course  the  ventral  portion  of  the  loop.  The  two  sacral 
myotomes  can  be  traced  into  the  foot,  the  first  giving  rise 


I 


rilK    I.IMII    MUSCl.KS.  235 

to  the  tnusculalurc  of  the  inner  portion  and  the  second  to 

// 


Fir  117  Kxtkknai,  Si^rface  of  thk  Os  Innominatum  showinc  tiih 
Attacmmknt  of  MrscuEs  and  the  Zones  Sii'i-lied  bv  the  \  akiois 
Nerves. 

12,  Twflftli   thoracic   nerve;    /    to    V,  lumbar   utrvis;    1    and   2,    ^acral 

nervi's.  -  t  l!«lk.) 

that  of  the  outer  portion,  and,  extending  to  the  tips  of  the 
toes,  they  are  reflected  upon  the  plantar  surface  and  so 


^36 


TlIK    DKVKJ.OI'MENT    OF    THE    HUMAN    IJODV. 


loop  upward  on  the  posterior  surface  of  the  leg  toward 
their  point  of  origin  froiTi  the  trunk. 

In  a  transverse  section  through  any  part  of  the  limb, 


Fig.  118.     Skctions  TiiKonin  (.1)  the  Tiii'-ii  .\ni>  (H)  thk  Cai.k  show- 
i.NT,  THK  Zones  Si  pi'Lied  bv  the  Nkkves.     The  Nerves  .vre  Nim- 

BEREO     I.\     Co.NTINlATK).-;     WITH     THE     ThoR.\CIC     vSERIES. — (.4     lljtir 

Hoik.) 


accordingly,  each  myotome  concerned  will  be  cut  twice, 
once  in  the  descending  (dorsal)  and  once  in  the  ascending 
(ventral)  portion  of  the  loop,  the  arrangement  found  being 


IMF.    I.IMr.    MISC  IKS. 


237 


thut  represented  in  l-i^.  •  iH.  The  nuxlif.eatmns  under- 
gone by  the  various  nnototnes  throughout  the  course  of 
tlieir  loops  resemble  those  :•:   ■uly  described  as  weurrniK  in 

the  trunk  nivotonies.  Thus,  each  of  the  muscles  repre- 
sented in  I'i-.  1 18.  H,  is  formed  by  the  fusion  of  elements 
derived  from  two  or  more  myotomes;  the  soleus  and  g^i!*- 
trocnemius  represent  deep  and  superficial  layers  formed 
from  the  same  mvotomes  by  a  horizontal  (tangential) split- 
ting, these  same  muscles  contain  a  portion  of  the  second 


K,..      11<)        SlXTlON    THKOK.H    THK    llM'KK    I'ART    OK    TllK    ArM    SIIOWIN.; 

Ti!K  Zonks  ,Si  ituied  by  thk  Nkkvks. 
Sv   t..   :v,   Wntnil   l.ranches;   5,/  U>  8,/,  d..rsal   brancl.i-s  of  the   cervical 

nerves.     {Hulk.) 

sacral  myotome  which  overlaps  muscles  composed  only  of 
higher  myotomes,  and  the  intermuscular  septum  between 
the  peroncus  brevis  and  the  flexor  longus  hallucis  repre- 
sents a  portion  of  the  third  sacral  myotome  which  has 
degenerated  into  connective  tissue. 

A  similar  arrangement  occurs  in  the  myotomes  entering 
into  the  formation  of  the  musculature  of  the  upper  limb 
These  are  the  fourth,  fifth,  sixth,  seventh,  and  eighth  cer- 
vical and  the  first  thoracic  myotomes,  and  of  these  only 


'  .^?v. 


238 


THE    DEVELOPMENT    OF    THE    HUMAN    BODY. 


the  eighth  cervical  and  first  dorsal  extend  as  far  as  the  tips 
of  the  fingers.  The  arrangement  of  the  myotomes  in  the 
upper  part  of  the  brachium  may  be  seen  from  Fig.  119,  in 
connection  with  which  it  must  be  stated  that  the  fourth 
cervical  myotome  does  not  extend  down  to  the  level  at 
which  the  section  is  taken  and  that  the  ventral  portion  of 
the  loop  of  the  eighth  cervical  and  both  portions  of  that 
of  the  first  dorsal  are  represented  only  by  conrective 
tissue  in  this  region. 


I 


i    ii 


LITERATURE. 

C    R.  B.\Ri)EEN  ANU  W.  H.  Lewis:  "Development  of  the  Limbs,  Body- 
wall,  and  Back  in  Man,"  The  American  Journal  of  Amit.,  1,  1901. 
K.    BardelEben:   "Miiskel  und  Fascia,"  Jenaische  Zeitschr.   jiir  Satur- 

•iVissensch.,  XV,  1882. 
L.   Bolk:  "Bezielmngen  zwischen    Skelett,  Muskiilatur  und  Xerven  dcr 

Extremitiiten,    dargelegt   am    Beckengiirtel,    an   dessen    Muskulatur 

sowie  am  Plexus  lumbosacralis,"  Morphol.  Jalirbuch,  x.xr,  1894. 
L.  Bulk:  "  Rekonstruklion  der  Segmentirung  derGliedmassenmuskulatur 

dargelegt  an  den  Muskeln  des  Oberschenkcls  und  des  Schultergiirtels," 

Morphol.  Jahrbuch,  xxii,   1895. 
L.   Bolk:  "Die  Sklerozonie    des    Humerus,"   Morphol    Jahrbuch,  xxiii, 

1896. 
L.   Boi,k:  "Die  Segmentaldifferen/.ierung  des  menschlichcn  Rumpfes  und 

seiner  Kxtremitaten,"  i,  Morphol.  Jahrbuch,  xxv,    1898. 
W    P.   Herringham:  '  The  Minute  .Anatomy  of  the  Brachial  Plexus," 

ProcciJini^s  oj  the  Royal  Soc.  London,  XLI,   1886. 
W.  H.  Lewis:  "The  DevcIo[)incnt  of  the  .\rm  in  Man,"  Amcr.  Jour,  oj 

Anal.,  I,  1902. 
J.    B.   MacCali.i'm:  "()n  the   Histology  and   Histogenesis  of  the   Heart 

Muscle-cell,"  Aiuit.   .Amrigrr,   xiii,   1897. 
J.    B.   MacCallum:  "On  the   Histogenesis  of  the  Striated   Muscle-fiber 

pnd  the  (Irowth  of  the   Human  Sartorius  Muscle,"  Johns  Hopkins 

Hospital  Bulletin,  1898. 
F.  P.  Mall:  "Development  of  the  \entral  Abdominal  Walls  in  Man," 

Journ.  oj  .Morphol.,   xiv,    1898. 
.\.  Meek:  "Preliminary  Note  on  the  Post -embryonal  History  of  Striped 

Muscle-fibers  in   Mammalia,"   .\nat.   Anzeiger,   xiv,    1898.     (See  also 

Anat.  Anzrigrr,  xv,   1899.) 
B.    MoRPURGo:    "Ueber   die    post-embryonale     Hntwickelung   der   cpier- 

gestreiften  Muskel  von  weissen  Ratten,"  Anat.  .Ameiger,  xv,  1899. 


•1 


•  I 


» 


LITERATURE. 


239 


I  PoPOWSKv:"Zur  Kntwicklungsgeschichte  des  N.  facialis  beim  Men- 
I.  Po'owsKV:  -Zur  KntwickeU.n«s,c.schich.e  der  Danunnu.skulutur  bam 
L    RHTHK    '  IHT   ..ripheren  Verlauf  der  n.otorisc-hcn  Kachen-  und  Oau- 

C    S^^u  v-ox:  • 'Xotes  ..n  ,1.  Arrangement  ..f  S-me  NW..  l'.i>ers 

in  the  Lun^ho-sacral  Plexus."  J..n,.,.l  of  /'/,..„./..  xjn    1S>2^ 
,    H   S,  TTON-  "l,i«anu-nts.  their  Nature  and  Mor,,l...lu5;y."  I.-ndnu.  180,. 


riib 


f 


CHAPTER    IX. 

THE    DEVELOPMENT     OF   THE    CIRCULATORY 
AND  LYMPHATIC  SYSTEMS. 

At  present  nothing  is  known  as  to  the  earliest  stages  of 
development  of  the  circulatory  system  in  the  human  em- 
bryo, but  it  may  be  supposed  that  they  resemble  in  their 
fundamental  features  what  has  been  observed  in  such 
forms  as  the  rabbit  and  the  chick.  In  both  these  the  sys- 
tem originates  in  two  separate  parts,  one  of  which,  located 
in  the  embryonic  mesoderm,  gives  rise  to  the  heart,  while 
the  other,  arising  in  the  extra-embryonic  mesoderm,  forms 
the  first  blood-vessels.  It  will  be  convenient  to  consider 
these  two  parts  separately,  and  the  formation  of  the 
blood-vessels  may  be  first  described. 

In  the  rabbit  the  extension  of  the  mesoderm  from  the 
embryonic  region  where  it  first  appears  over  the  yolk-sac 
is  a  gradual  process,  and  it  is  in  the  more  peripheral  por- 
tions of  the  layer  that  the  blood-vessels  first  make  their 
appearance.  They  can  be  distinguished  before  the  split- 
ting of  the  mesoderm  has  been  completed,  but  are  always 
developed  in  that  portion  of  the  layer  which  is  most  inti- 
mately associated  with  the  yolk-sac  and  consequently 
becomes  the  splanchnic  layer.  The  first  indication  of  the 
vessels  is  the  appearance  in  the  peripheral  portion  of  the 
mesoderm  of  cords  or  minute  patches  of  spherical  cells 
(Fig.  1 20,  A  ).  These  increase  in  size  by  the  division  of  the 
cells  and  by  their  separation  from  one  another  (Fig.  120, 
B),  a  clear  fluid  appearing  in  the  intervals  which  separate 
them.     Soon  tiie  cells   surrounding  each  cord   arrange 

240 


THK    Bl.OOn. 


241 


themselves  to  form  an  enclosing  wall,  and  the  cords,  m- 
creasing  in  size,  unite  together  to  form  a  networl  of  ves- 
sels in  which  float  the  spherical  cells  which  may  now  be 
known  as  erythrocytes.  Viewed  from  the  surface  at  tins 
stage  a  portion  of  the  vascular  area  of  the  mesoderm 
would  have  the  appearance  shown  in  Fig.  121,  reveaimg  a 
dense  network  of  canals  in  which,  at  intervals,  are  groups 
of  ervthrocvtes  adherent  to  the  walls,  constituting  what 


itl 


T,-,,;     1 70 —Transverse   Section  through  the   Area   VAscyuosA   of 
Rabbit   Embryos   showinc;   the  Transformation   ok   Mesoderm 
Cells  into  the  Vascular  Cords. 
Ec,  Ectoderm;  En,  endoderm;  Mc,  mesoderm.— (wh  dcr  Slricht.) 

have  been  termed  the  hlood-islands,  while  in  the  meshes  of 
the  network  unaltered  mesoderm  cells  can  be  seen,  form- 
ing the  so-called  substance-islands. 

At  the  periphery  of  the  vascular  area  the  vessels  ar- 
range themselves  to  form  a  sinus  terminalis  enclosing  the 
entire  area,  and  the  vascularization  of  the  splanchnic 
mesoderm  gradually  extends  toward  tlie  embryo.  Reach- 
ing it,  the  vessels  penetrate  the  embryonic  tissues  and 
eventually  come  into  connection  with  the  heart  which  has 


I 


I  1 


W- 


24: 


THE    DEVELOPMENT   OF     THE    HUMAN    JtODV. 


already  differentiated  and  has  begun  to  beat  before  the 
connection  with  the  vessels  is  made,  so  that  when  it  is 
made,  the  circulation  is  at  once  established.  Before, 
liowever,  tlie  vascularization  reaches  the  embryo  some  of 

the  canals  begin  to  en- 


^■ 


large  (Fig.  122, /I),  pro- 
ducing arteries  and  veins, 
the  rest  of  the  network 
forming  capillaries  unit- 
ing these  two  sets  of 
vessels,  and,  this  process 
continuing,  there  are 
eventually  differentiated 
a  single  omphalo-mesen- 
teric  (vitelHne)  artery  and 
two  omphalo  -  m  -^enteric 
(vitelline)  veins  (Fig.  122, 

B). 

In  the  human  embryo 
the  small  size  of  the 
yolk-sac  permits  of  the 
extension  of  the  vascu- 
lar area  over  its  entire 
surface  at  an  early  pe- 
riod, and  this  condition 
has  already  been  reached 
in  the  earliest  stages 
known  and  consequently 
no  sinus  terminalis  such 
as  occurs  in  the  rabbit  is 
visible.  Otherwise  the  conditions  are  probably  similar  to 
what  lias  been  described  above,  the  first  circulation  de- 
veloped being  associated  with  the  yolk-sac. 

The  Formation  of  the  Blood.—  The  erythrocytes,  which 


Vir..  121.— SuKi-Acii  Viiiw  or  a  I'ok- 

Tiox  OK  THE  Akica   Vascluosa  or 

A  Chick. 
The  vascular  network  is  rciiresented  by 

tliesliaded])orti()n.   Hi,  IJlood-island ; 

5/,  sut)stancc-island.  ^(/>/vv<'.) 


THE    inXH)D. 


243 


are  the  first  blood-corpuscles,  arc  all  nucleated  and  are  for 
a  time  the  only  cells  occurring  in  the  blood,  though  later 
other  cells,  arising  in  tissues  exterior  to  the  blood-vessels, 
make  their  way  n'Ao  the  vessels,  forming  leukocytes.  From 
their  very  first  formation  then  tlie  red  (erythrocytes)  and 
white  (leukocytes)  blood-corpuscles  have  a  difi"erent  ori- 
gin, and  they  remain  distinct  throughout  life,  (me  form 
never  becoming  converted  into  the  other. 


1,„;   P2  —The  Vascular  Areas  ok  Rabbit  Ivmbkyos.     In  n  the  Veins 

ARE    represented    by    HuACK    AM)    THE     NETWORK    IS    OMITTED. 

{van  HomUn  and  Juliii.) 


So  long  as  the  formation  of  blood-vessels  is  taking  place 
in  the  extra-embryonic  mesoderm,  so  lout,  are  new  ery- 
throcytes being  differentiated  from  the  mesoderm.  But 
whether  the  formation  of  blood-vessels  within  the  embryo 
results  from  a  differentiation  of  the  embryonic  mesoderm 
in  situ,  or  from  the  actual  ingrowth  of  vo'^sels  from  the 
embryonic  regions  (His),  is  as  yet  uncertain,  and  hence 
it  is  also  uncertain  whether  erythrocytes  are  differentiated 


244 


THE    DEVELOl'MENT    OF     THE    HUMAN    BODY. 


<  :  * 


from  the  embryonic  mesoderm  or  merely  pass  into  the 
embryonic  region  from  the  more  peripheral  areas.  How- 
ever this  may  be,  it  is  certain  that  the  erythrocytes  in- 
crease by  division  in  the  interior  of  the  embryo,  and  that 
there  are  certain  portions  of  the  body  in  which  these  divi- 
sions take  place  most  abundantly,  partly,  perhaps,  on 
account  of  the  more  favorable  conditions  of  nutrition 
which  they  present  and  partly  because  they  are  regions 
where  the  circulation  is  sluggish  and  permits  the  accumu- 
lation of  erythrocytes.  These  regions  constitute  what 
have  been  termed  the  hcematopoietic  organs,  and  are  espe- 
cially noticeable  in  the  later  stages  of  fetal  life,  diminishing 
in  number  and  variety  about  the  time  of  birth.  It  must 
be  remembered,  however,  that  the  life  of  individual  cor- 
puscles is  comparatively  short,  their  death  and  disintegra- 
tion taking  place  continually  during  the  entire  life  of  the 
individual,  so  that  there  is  a  necessity  for  the  formation  of 
new  corpuscles  and  for  the  existence  of  haematopoietic 
organs  at  all  stages  of  life. 

In  the  fetus  erythrocytes  in  process  of  division  may  be 
found  in  the  general  circulation  and  even  in  the  heart 
itself,  but  they  are  much  more  plentiful  in  places  where 
the  blood-pressure  is  diminished,  as,  for  instance,  in  the 
larger  capillaries  of  the  lower  limbs  and  in  the  capillaries  of 
all  the  visceral  organs  and  of  the  subcutaneous  tissues. 
Certain  organs,  however,  such  as  the  liver,  the  spleen,  and 
the  bone-marrow,  present  especially  favorable  conditions 
for  the  multiplication  of  the  blood-cells,  and  in  these  not 
only  are  the  capillaries  enlarged  so  as  to  afford  resting- 
places  for  the  corpuscles,  but  gaps  appear  in  the  walls 
of  the  vessels  through  which  the  blood-elements  may 
pass  and  so  come  into  intimate  relations  with  the  actual 
tissues  of  the  organs  (Fig.  123).  After  birth  the  haemat- 
opoietic function  of  the  liver  ceases  and  that  of  the  spleen 


THE    BLOOn. 


245 


( 


becomes  limited  to  the  formation  of  white  corpuscles, 
though  the  complete  function  may  be  re-established  in 
cases  of  extreme  ansemia.      The  bone-marrow,  however, 
retains       the       func- 
tion completely,  being 
throughout     life     the 
seat  of    formation   of 
both    red    and   white 
corpuscles,  the  lymph- 
atic   nodes  and    folli- 
cles,   as    well    as    the 
spleen,  assisting  in  the 
formation  of  the  latter 
elements. 

Until  about  the  sec- 
ond month  of  develop- 
ment the  erythrocytes 
and  leukocytes  are  the 
only  elements  found  in 
the  blood,  and  in  the 
haematopoietic  organs 
they  may  be  observed  in  active  mitosis.  In  addition 
other  cells,  having  the  same  general  appearance  as  the 
erythocytes  but  lacking   hemoglobin,  also   occur,   and 

these,  which  may  be  termed 
crythrohlasfs,     produce     by 
division     erythrocytes     in 
which    hcEmoglobin   gradu- 
ally appears.    After  the  sec- 
ond month, however,  a  third 
form  of  blood-elements  ap- 
pears in  the  form  of  non-nucleated  discs  containing  haemo- 
globin, and  these  may  be  termed  crythroplastids.     They 
are  derived  frrm  the  erythrocytes,  whose  nuclei,  originally 


Fic.  12.^.  -Section- OK  .\  Portion  dk  thk 
I.ivER  OF  A  Rabbit  Kmbryo  ov  5  mm. 

r,  Erythrocytes  in  the  Hvcr  substance 
'  and  in  a'  capilla.y;  /(,  hepatic  cells.— 
(-.(!»  di-r  Stricht.) 


Fic.  124.— Staoes  I.N  THE  Tkans- 

l-ORMATION  OF  AN  KrvTHROCYTE 
INTO  AN  ERYTHROPLASTH).  -  {vhh 
dcr  Slricht.) 


HMl 


!■■ 


246 


THE    DEVELOPMENT    OF     THE    HUMAN    BODY. 


reticular  in  structure,  gradually  conciense  to  become  spheri- 
cal, deeply  staining  masses,  and  are  finally  completely 
extruded  from  the  cytoplasm  (Fig.  124).  The  cast-off 
nuclei  undergo  degeneration  and  phagocytic  absorption 
by  the  leukocytes,  and  the  masses  of  cytoplasm  pass  into 
the  circulation,  becoming  more  and  more  numerous  as 
development  proceeds,  until  finally  they  arc  the  only  haemo- 
globin-containing elements  in  the  blood  and  form  what 
are  properly  termed  the  red  blood-corpuscles.  In  the 
later  fetal  and  the  post-natal  stages  erythrocytes  are  to  be 
found  only  in  the  red  bone-marrow. 

In  the  formation  of  the  new  leukocytes  there  is  a  ten- 
dency for  the  dividing  cells  to  collect  in  more  or  less  defi- 
nite groups  which  have  been  termed  germ-centers  (Flem- 
ming).  The  new  cells  when  they  first  pass  into  the  cir- 
culation have  a  relatively  large  nucleus  surrounded  by  a 
small  amount  of  cytoplasm,  and,  since  they  resemble  the 
cells  found  in  the  lymphatic  vessels,  are  termed  lympho- 
cytes. In  the  circulation  the  nuclei  become  larger  and  the 
cytoplasm  more  voluminous  and  amcEboid,  the  cells  being 
then  known  as  mononuclear  leukocytes,  and  transitional 
forms  lead  from  these  to  still  larger  cells  with  irregularly 
lobed  or  branched  nuclei,  the  polymorphonuclear  leuko- 
cytes, while  these  again  seem  to  lead  to  poly  nuclear  cells. 
It  is  probable  that  these  various  kinds  of  cells  stand  in 
genetic  relation  to  one  another,  the  polymorphonuclear 
and  polynuclear  forms  perhaps  representing  the  com- 
mencement of  the  degeneration  and  breaking  down  of  the 
elements. 

In  the  fetal  litematopoietic  organs  and  in  the  bone-mar- 
row of  the  adult  large,  so-called  r/Zuw/cc//^  are  found,  which, 
although  they  do  not  enter  into  the  general  circulation,  are 
yet  associated  with  the  development  of  the  blood-cor- 
puscles.    These  giant-cells  as  they  occur  in  the  bone- 


THE  ni.oon. 


247 


marrow  arc  of  two  kinds  which  seem  to  be  quite  distinct, 
although  both  are  probably  formed  from  leukocytes.  In 
one  kind  the  cytoplasm  contains  several  nuclei,  whence 
they  have  been  termed  polycaryocytes,  and  they  seem  to 
be  the  cells  which  have  already  been  mentioned  as  osteo- 
clasts (p.  180).  In  the  other  kind  (Kig.  125)  the  nucleus 
is  single,  but  it  is  large  and  irregular  in  shape,  frequently 
appearing  as  if  it  were  producing  buds.  These  mega- 
caryocytes  appear  to  be  phagocytic  cells,  having  as  their 


l''iG.  125. — Portion  ok  a  Section  from  the  Liver  of  an  Umbryo  Cat 

OK  2.7    .VIM.   SHOWINO   A   MEdALARVOCVTE   SURROUNDED   BY   RrYTHRO- 
CYTES  IN   A    BLOOO-VESSEU.  — (//(Wi7/.) 

function  the  destruction  of  degenerated  corpuscles  and  of 
the  nuclei  of  the  erythrocytes. 

Little  is  certainly  known  as  yet  as  to  the  origin  of  the 
blood-platelets,  thougli  the  most  plausible  suggestion  is 
that  they  are  the  fragmented  nuclei  of  broken-down 
leukocytes. 

The  question  of  the  origin  of  the  various  forms  of  blood 
elements  is  a  ver\-  difficult  ouu,  and  the  opinions  of  sonic  ob 
servers  are  very  different  from  some  of  the  statements  made 
above.     Thus  it  has  been  maintained  that  the  nuclei  of  the 


iai 


1 


t;  I 


248  THE    OEVELOPMENT   OF    THE    HUMAN    HODV. 

I  A  ;«  flie  formation  of  t-rythro- 

ptetids,  hul  undiTRo  a  S8«J  "',,„„„.  |,„.„nu.  transformed 
[hat  mc»-nchyn.c  "•'*"','.'■„"  p,,,,n,,ckar  and  polynncloar 
into  leukocytes;  that  ""j„I'-,"3ismtesration.  but  represent 
kukocvtes  are  n..tstagi-s  leading  to  dtsns  .^  ^^^  ,.^,^, 

stages  of  a.nUot,ed.vis.o,.,etc^    ,"«.  various  ideas  and    '-e 
Svatn'  tave  W  chosen  .or  presentat.on. 

'--— ,r:lrt;^"p:::l--"- 
T  r  1  Tr  con.;  ."1  cont»ct  in  the  median  line, 
which  only  later  come  into  ,„_,„:„,  of  the  cmbry- 

On  each  side  of  the  body  near  ''^  ^^^'"^.^^p^jeetinK 
onic  area  a  fold  of  the  'P'^-'f^'P^^  .j'^''^  ,"  a  very  thin- 
i,„o  the  calomic  cavity  ''"'  """ '^"^/°„' "„„  o/its  in- 

nermost  cells  (Imr.   '^I^'.    ,„'  ,„„,„,,■„,„)  of  the  heart, 
portion  of  the  '"'^^^^^^  Zi..,iu,»,     A 
and  each  sac  part  of  its  emiotl.u.u  proceeds, 

,„  constriction  of  0.ee,n.ry.r.t|.c  >^^^^  ^^.^ 

the  two  folds  are  gradual  y  oro  k  ^^^ 

sac  and  in  what  w.ll  later  be  "«^  «;;';^,,  ^.„i,„  „ave 

■  ^uts  rar-inals,  unit^  -.ethe^to  open  in^^^^^^^^^^^ 

ike  structure,  the  s.jms  --"7-.  ^"'';^,;  Xrior  end  of 
posterior  end  of  the  ^l^^^^J^^U^  is  con- 
tl,ecyUndertapersoff  tofor.     he.  ta^vngeal 


THE    HEART. 


249 


blood  accordingly  opens  into  the  posterior  end  of  the  heart 
tube  and  flows  out  from  its  anterior  end. 


en 

Fig.    126.— Di.\f;R.\MS   1ulustr.\tino   the    Form.\tion   of   the    Heart 

IN  THE  (•!  INEA-PIO. 

The  mesoderm  is  represented  in  black  and  the  endocardium  by  a 
broken  line,  am,  Amnion;  en,  endoderm;  //,  heart;  i,  digestive  tract. 
— {After  Stralil  and  Carius.) 

The  simple  cylindrical  form  soon  changes,  however,  the 

heart  tube  in  embryos  of  2.15  mm.  in  length  having  be- 
21 


i^%\ 


250 


THE    DKVEI.Ol'MKNr    OF     i     > 


ii(!\i,  V  nonv, 


come  bent  upon  itself  into  a  nvulun  S  shaiMcl  curve 
(Fie  .27).  Dorsallv  and  to  the  I  It  i-  H,.  lower  end  mto 
which  the  sinus  venosus  opens,  uid  f..--,  .his  the  heart 
lube  ascends  somewhat  and  then  bends  ...  as  to  pass  at 
first  ventrally  and  then  downward  and  to  the  right,  where 
it  aL^ain  bends  at  first  dorsally  and  then  anteriorly  to  pass 
over  into  the  aortie  bulb.     The  portion  of  tlu-  curve  whieh 


1 


I    I 


,.  „  Vv    1  '8      Heart   of  Ivmbh  .0  of 

Fi(..    127. -Heart  ..f    Kmbkno  _^  -^^  ^^^.^.  j..^„^,  thk  1*orsai. 

OF    2.1.S    MM.,   FROM    A  RKCON-  v^,"rF\CE 

'"'■'T"'\          .-   hull..  ,/  /n-.DucU.sCuvieri:/.4.1eft    .rick-; 

a,    Aunde;  ah    lu.rt  c   >.«,'•  ,1,  riKhl  auricle;  :/,  i"K'.l^.-       '": 

diaphragm  ;r/r,  duct  us  Cuvicn,  •  ,^k.^    ventricle;  vu,    uiM...i.al 

/,  liver;  r,  venlnclc;  :;,  JUU"-  *   •   '     ^^^     ^^ 

lar  vein;  vu,   umbilical   vein.  ^«-i 

-(///v  ) 

lies  dorsally  and  to  the  left  is  destin -d  to  ^J^^^ '  '^^^^ 
aurieles,  the  portion  which  passes  from  rtght  to  left  rcpr. 
sents  the  future  left  ventricle,  while  the  succeeding  1  .r  o. 
represents  the  right  ventricle.  In  later  stages  (Fi,  128) 
the  left  ventricular  portion  drops  downward  m  h  -■  -■' 
the  auricular  portion,  assuming  a  more  l-"-"^\  ^;;;;  " 
tion  while  the  portion  which  represents  the  right  ven 


% 


THE    IlKART. 


^51 


f 


trick-  is  drawn  forward  so  a^  to  li.   m  llir  same  plane  as 
the  left. 

At  the  same  time  tv^o  small  outf  -.chii  <level<  . j  from 
the  auricular  part  of  the  heart  and  iorni  ic  firs  indica- 
tions of  the  two  auri.  les.  .\  ^  development  pn  resses, 
these  increa-^  in  size  ti  form  lari,a»  p»M2ches  opening  into  a 
common  auncular  <  ;ui  '  (I*ii(  i-'<>)  wiiich  is  directly  con- 
tinuous with  the  left  ventrKle.  and  as  the  enlargement  of 
the  pouches  continues  their  openinj;s  into  the  canal  en- 
large, until  finally  tlu 
pouches  beeomt  conti^ 
uous  with  (»ne  anothei 
fo'ining  a  single  la;,i(< 
sa^-,  ami  the  auricular 
canal  becomes  reduced 
to  a  short  tube  u  liicli  is 
slightly  invaginated  into 
the  "en*  ricle  ( Kig .  ;  t,(>  t 

In  iht    mean  time  th.. 
sinus  venosus,  vvhicii  wa 
originally    an    oval       sc 
and  opened  into  the  au- 
ricular  canal,   ha'-    eloi 
j^atel  transversely      Mtil 
it  h„,s  issumed  th(   form  of  a  cr    cent  whose 
'on  tact  with  the  wa  Is   if  tlie  auru  les   and 
»  the  htart  has  verged   towaid  the     ight,  unti.   it  is 
ated  entinly  within  the  area  of  tin.  right  auricle.     As 
le  eniari,'ement  of  th'/  auricles  continues,  the  right  horn 
nd  median  portion  of  the    resceiit  are     r  (dually  taken 
i_p  into  their  walls,  so  tha      he  various  vei-         licli  origi- 
nally ofjened  into  the  sin    -  now  open  diiectis    Inlo  liie 
right  auricle  by  a  single  op.  ning,  guarded  by  a  projecting 
fold  which  is  continued  upon  the  roof  of  the  auricle  as  a 


l*iG    1_"»      Heart 
MM     Si;en    fro       1 

Si  rOHTl.Y    FROM    ABi 


IHiVO    '^F    "^ 

Fh    mt    ami 


ttV  IS 

nine: 


252 


THE    DEVELOPMENT   OF    THE    HUMAN    nODV. 


muscular  ridge  known  as  the  septum  spurium  (Fig.  130. 
sp).  The  left  horn  of  the  crescent  is  not  taken  up  into 
the  auricular  wall,  but  remains  upon  its  posterior  surface 
as  an  elongated  sac  forming  the  coronary  sinus. 

The  division  of  the  now  practically  single  auricular  cav- 
ity into  the  permanent  right  and  left  auricles  begins  with 
the  formation  of  a  falciform  ridge  running  dorso-ventrally 


'''^•//..//V"''''^''^'^, 


Fio.   130,     Inner  vSikface  of  the  Heart  ok  an  Embryo  of  10  mm. 

al     .\uricul()  ventricular   thickeninR;    sh,    septum    spurium;     va,    septum 

primuin;  sv,  sci)tum  ventriculi;  re,  Kustacluan  valve.     (Hjv.; 

across  the  roof  of  the  cavity.  This  is  the  auricular  septum 
or  septum  primum  (Fig.  130,  ss),  and  it  rapidly  increases 
in  size  and  thickens  upon  its  free  margin  which  reaches 
almost  to  the  upper  border  of  the  short  auricular  canal 
(Fig.  132).  The  continuity  of  the  two  auricles  is  thus 
almost  dissolved,  but  is  soon  re-established  by  the  forma- 
tion in  the  dorsal  part  of  the  septum  of  an  opening  which 


^ 


THE    HEAKT. 


253 


Si  SzjL^vrs 


soon  reaches  aVonsiderable  size  and  is  known  as  the  jora- 
soon  ^eac»e^  J^  ^^  ^j^^  auricular  septum, 

Z;:^^^'^:^^^o.. rid.e  appears  in  the  roof  and 
^^ntfal  wall  of  the  right  auricle.     Thjs  ^  securuiunr 
(S )  is  from  the  beginning  very  much  thicker  than  trie 
luri  ular Teptum,  and  its  free  end.  arching  around  tlie 
ventraUdge'and  floor  .f  the  foramen  ovale,  becomes  con- 
tinuous with  the  left  lip  of  the 
fold  which  guards  the   opening 
of  the  sinus  venosus  and  with 
this  forms  the  annulus  of  Vieus- 
sens  of  the  adult  heart. 

When  the  absorption  of   the 
sinus  venosus  into   the  wall  of 
the  right  auricle  has  proceeded 
so  far  that  the  veins  communi- 
cate directly    with  the  auricle, 
the  vena  cava   superior   opens 
into  it  at  the  upper  part  of  the 
dorsal  wall,  the  vena  cava  in- 
ferior more  laterally,  and  below 
this  is  the  smaller  opening  of 
the  coronarj  sinus.     The  upper 
portion  of  the  right  lip  of  the 
fold  which  originally  surrounded 
the  opening  of  the  sinus  venosus, 
together  with  the  septum  spu- 

rium,  gradually  disappears;  the  lower  portion  persist.s, 
however,  and  forms  ( i )  the  Eustachia u  valve  ( Fig.  131,  V  e) , 
guarding  the  opening  of  the  inferior  cava  and  directmg 
the  blood  entering  by  it  toward  the  foramen  ova  e,  and  (2) 
the  Thebesian  valve,  which  guards  the  opening  of  the  coro- 
nary sinus.  At  first  no  veins  communicate  with  the  left 
auricle  but  on  the  development  of  the  lungs  and  the  es- 


Kio     1.^ I.  -Heart   of    Km- 

BRVO     OI-     10.2     CM.     FROM 
WHICH  H.\I.FOFTHE  RlCHT 

AiBlCi.E    H.\s    HEEN  RE- 

MOVEP. 

/„  iMiranien  ovale;  /xj,  pul- 
'monary  artery ;.S,,  septutn 
tiriinuni ;  .S.„  septutn  secun- 
iliini;  Sa,  systemic  aorta; 
1',  rijjlit  ventricle;  rci, 
atid  res  inferior  and  supe- 
rior ven;e  cava-;  V'»',  Eus- 
tachian valve. 


:  i 


254 


THE    OEVELOPMENT    OF     THE    HUMAN    BODY. 


tablishment  of  their  vessels,  the  puhnonary  veins  make 
connection  with  it.  Two  veins  arise  from  each  lung,  and 
as  they  pass  toward  the  heart  they  unite  in  pairs,  the  two 
vessels  so  formed  again  uniting  to  form  a  single  short  trunk 
which  opens  into  the  upper  part  of  the  auricle  (Fig.  132, 
Vep).  As  is  the  case  with  the  right  auricle  and  the 
sinus  venosus,  the  expansion  of  the  left  auricle  brings 
about  the  absorption  of  the  short  single  trunk  into  its 
walls,  and,  the  expansion  continuing,  the  two  vessels  are 
also  absorbed,  so  that  eventually  the  four  primary  veins 
open  independently  into  the  auricle. 

While  the  auricular  septa  have  been  developing  there 
has  appeared  on  the  dor  ,al  wall  of  the  auricular  canal  a  tu- 
bercle-like thickening  of  the  endocardium,  and  a  similar 
thickening  also  forms  on  the  ventral  wall.  These  endo- 
cardial cushions  increase  in  size  and  finally  unite  together 
by  their  tips,  formmg  a  complete  partition  dividing  the 
auricular  canal  into  a  right  and  left  half  (Fig.  132).  With 
the  upper  edge  of  this  partition  the  thickened  lower 
edge  of  the  auricular  septum  unites,  so  that  the  separa- 
tion of  the  auricles  would  be  complete  were  it  not  for  the 
foramen  ovale. 

While  these  changes  have  been  iaking  place  in  the  au- 
ricular portion  of  the  heart,  the  separation  of  the  right  and 
left  ventricles  has  also  been  progressing,  and  in  this  two 
distinct  septa  take  part.  From  the  floor  of  the  ventricu- 
lar cavity  along  the  line  of  junction  of  the  right  and  left 
portions  a  ridge,  composed  largely  of  muscular  tissue, 
arises  (Figs.  130  and  132),  and,  growing  more  rapidly  in  its 
dorsal  than  its  ventral  portion,  it  comes  into  contact  and 
fuses  with  the  dorsal  part  of  the  partition  of  the  auricular 
canal.  Vcntrally,  however,  the  ridge,  known  as  the  ven- 
tricular septum,  fails  to  reach  the  ventral  part  of  the  par- 
tition, so  that  an  oval  foramen,  situated  just  below  the 


II  :t 


t  I 


THE    HEART. 


'55 


point  where  the  aortic  bulb  arises,  still  remains  between 
l^^rtwo  ventrides.  This  opening  is  finally  closed  by  what 


•  M- 


En.s 


SM 


V,,.    1,^1  -Section  through  a  Reconstruction  ok  the  Heart  of  a 
Rabbit  Embryo  uV  10.1  mm. 

\d  ind  Ad  Riklit  and  .U,  left  auricle;  Uw,  and  liw,,  lovyer  ends  of 
the  ridies  which  divide  the  aortic  bulb;  En,  endocardial  cushion 
SramfE^,  thickenings  of  the  cushion;  la,  inteiauncular  and 
5;  interventricular  communication ;  .„  septum  pnmum ;  SJ  right  and 
SX  left  horn  ..f  the  sinus  venosus;  S.n,  yen  ricu  ar  ^'l^tum  SA. 
ooeninii  of  the  sinus  venosus  into  the  auricle;  V(/,  '^^'ght  ami  i  . 
ffwntricle;  VV/.  jugular  vein;  V,f,  pulmonary  vem;  I  rJ  and 
V't^/rVght  and  lefl  limbs  of  the  valve  guarding  the  opemng  of  the 
sinus  venosus, — {Horn.) 

is  termed  the  aortic  septum.  This  makes  its  appearance  in 
the  aortic  bulb  just  at  the  point  where  the  first  lateral 
branches  which  give  origin  to  the  pulmonary  arteries  (see 


4-^5 


•ft-V';' 


256  TICF.    I.EVF.1X>P.1F.NT   OF    TI.F    HUMAN    BOl.V. 

,  .64)  ari^.  and  is  forn.ed  by  the  fusion  of  the  free  edges 
of  two  ridse;  which  develop  on  opposite  sides  of  the  bulb^ 
PWit"  point  of  origin  it  gradually  extends  down  te^bub 
until  it  reaehes  the  ventricle,  vhere  .t  fuses  with  the  free 
edge  of  the  ventricular  septum  and  so  completes  the  sepa- 
ratTonof  the  two  ventricles  (Fig.  .33).  The  bulb  now  con- 
Ss  of  two  vessels  lying  side  by  side,  -d  owing  »„  he  po^ 
sition  of  the  partition  at  its  anterior  end,  one  of  these 
«  sels,  that  which  opens  into  the  right  ventricle  -s  con 
til  uous  with  the  pulmonary  arteries,  while  the  other 
which  opens  into 'the  left  ventricle,  is  continuous  w.th 
H  Irest  of  the  vessels  which  arise  from  the  forward  con- 
inuattn  of  the  bulb.      As  soon  as  the  development  of 
t  "e  plrtWon  is  completed,  two  grooves,  corresponding  in 
p tition  to  the  lines  of  attachment  of  the  Partm--   ^ 
inside  of  the  bulb,  make  their  appearance  on  the  outside 
Xadually  deepen  until  they  finally  -et  and    -vid 
the  bulb  into  two  separate  vessels,  one  of  which  is 
pulmonarv  aorl,  and  the  other  the  systemic  aorta^ 

In  the  .arlv  stages  of  the  heart's  development  the  mus 
cU  bll  es  which  compose  the  wall  of  the  ventricle  are 
ver    looselv  arranged,  so  that  the  ventricle  is  a  somewha 
spo ,«     ni.  ss  of  mu^ular  tissue  with  a  relatively  smaU 
cav  tv      As  .levelopmen.  proceeds  the  bundles  neares 
X  outer  surface  come  clos..  ,  ogether  and  form  a  compact 
la  e"   Uiose  on  the  inner  surl.ce,  -owcver  retaining  the, 
loose  arrangement  for  a  long.r  ..me  (^'g'^.    ""^  °;„ 
„1„.  of  .he  auricular  canal  tK-comes  prolomied  on  the  lei 
1^:L  one,  and  on  the  right  side  into  two  flaps  wh, 
oroiect  downward  into  .he  ventricular  cavity,  and  an  a  . 
Snal  flaparises,„  each  .defromthe  lower  edge^^^^^ 

^t?":;h:  a::;:xt:.::'^;=:;ig  :nd «;  mthe 

left      To  the  under  surfaces  of  the*  flaps  the  loosely 


Oi— 


Fav.d 


'~  •;;„fs;r:,.™^s;;;L  rv,i:f  ;";^™eS'^^° 


R  \HHITS  TO   SHOW 

OK  THE   Ar'RIClH.O-VICNTKlCrLAK   OKHICIv. 

Ao,  A«)rt;i;  Ar.p,  VVL\\wmM\^  ''.V^"^v',    ''' 


bull);   /•';.. i,   '"It-   'j    t''c 


A„ru,;  ,U.f,  ,,ul.n„narv  .ru.,v    ,'■;,;:;;";';;;.'■„ 'i",;;,, ru.ick.n- 

22  -57 


rfs.""  "•-  fv,*i  ^^^IBr'^'i' 


il  ' 


if   i 


258 


THE    DEVELOPMENT   OF     THE    HUMAN    BODY. 


arranged  muscular  trabeculaeof  the  ventricle  are  attached, 
and  muscular  tissue  also  occurs  in  the  flaps.  This  condi- 
tion is  transitory,  however;  the  muscular  tissue  of  the 
flaps  degenerates  to  form  a  dense  layer  of  connective  tis- 
sue, and  at  the  same  time  the  muscular  trabeculae  undergo 
a  condensation.  Some  of  them  separate  from  the  flaps, 
which  represent  the  auriculo-ventricular  valves,  and  form 
muscle  bundles  which  may  fuse  throughout  their  entire 
length  with  the  more  compact  portions  of  the  ventricular 
walls,  or  else  may  be  attached  only  by  their  ends,  form- 
ing loops ;  these  two  varieties  of  muscle  bundles  constitute 


mh' 


Fio.   134.— Diagrams  showino  the  Development  of  the  Auriclu)- 

VENTRICrUAK|VAUVES. 

ft,  Muscular  trabeculae;  cht,  chord;c  tendinesp;  mk  and  mk\,  valve;  />»(, 
musculus  papillaris;  tc,  columna-  earner ;  r,  ventricle.  —  (/•  rom  Ilcrtutg, 
after  Gegcnbaur.) 

the  columnce  carnea>  of  the  adult  heart.  Other  bundles 
may  retain  a  transverse  direction,  passing  across  the  ven- 
tricular cavity  and  forming  the  so-called  moderator  hands; 
while  others,  again,  retaining  their  attachment  to  the 
valves,  condense  only  at  their  lower  ends  to  form  the  mus- 
culi  papillares,  their  upper  portions  undergoing  conversion 
into  strong  though  slender  fibrous  cords,  the  chordce  ten- 
dinecE  (Fig.  134). 

The  endocardial  lining  of  the  ventricles  is  at  first  a  sim- 
ple sac  separated  by  a  distinct  interval  from  the  myocar- 


THE    HEART, 


259 


dium  but  When  the  condensation  of  the  muscle  trabeculae 
o  cur's  the  Endocardium  applies  itself  closely  to  the  irregu^ 
^r Trf ace  so  formed ,  dipping  into  all  the  -v^ces  betw^^^^^^ 
the  column,  carne.  and  wrapping  itself  around  U^e  mu  - 
culi  papiUares  and  chordae  tendmeae  so  as  to  orm  a  com 
Dlete  lining  of  the  inner  surface  of  the  myocardmm 

^  The  aorUc  and  pulmonary  senulunar  valves  make  the.r 
appearance,  befor^the  aortic  bulb  undergoes  Us  longUu- 
Tal  splitting,  as  four  tubercle-like  thtckemngs  of  con- 
nective tissue  situated  on  the  mner 
wall  of  the  bulb  just  where  it  arises 
from  the  ventricle.     When  the  di- 
vision of  the  bulb  occurs,  two  of 
the  thickenings,  situated  on  oppo- 
site  sides,    are    divided,   so   that 
both  the  pulmonary  and  systemic 
aorta?   receive    three    thickenings 
(Fig.  135).     Later  the  thickenings 
become  hollowed  out  on  the  sur- 
faces directed  away  from  the  ven- 
tricles  and  are  so  converted  into  the  pouch-like  valves 

°'c'L;tt  are  Heart  after  Birth.-TUe  heart  when  first 
for„.ed  lies  far  forward  in  the  neck  region  oH^- -bry°; 
between  the  head  and  the  anterior  surface  of  the  yolk-sac, 
and  from  this  position  it  gradually  recedes  unti  1   reach^^^ 
its  final  position  in  the  thorax.     And  not  only  doe       tlvu 
change  its  relative  position,  but  the  direction  of   ts  axes 
aUo  chlnge.      For  at  an  early  stage    the  ventricles  he 
l^ecly  in  front  of  (.  ...  ventrad  to)  the  auricles  and  not 
below  them  as  in  the  adult  heart,  and  this  primitive  con- 
dition is  retained  until  the  diaphragm  has  reached  its  final 

position  (see  p.  34^)-  .  .         .  „^^^t^„i 

In  addition  to  these  changes  in  position,  important 


Vic  135. — Diagrams  Il- 
lustrating THE  For- 
mation OK  THE  Semi- 
lunar V.\uvES.— C-rC- 
f^enbaur.) 


26o 


THE    DEVELOPMENT    OF    THE    HUMAN    BODY. 


changes  also  occur  in  the  auricular  septum  after  birth. 
Throughout  the  entire  period  of  fetal  life  the  foramen 
ovale  persists,  permitting  the  blood  returning  from  the 
placenta  and  entering  the  right  auricle  to  pass  directly 
across  to  the  left  auricle,  thence  to  the  left  ventricle,  and 
so  out  to  the  body  through  the  systemic  aorta  (see  p.  288). 
At  birth  the  lungs  begin  to  function  and  the  placental 
circulation  is  cut  off,  so  that  the  right  auricle  receives  only 
venous  blood  and  the  left  onjy  arterial ;  a  persistence  of 
the  foramen  ovale  beyond  this  period  would  be  injurious, 
since  it  would  permit  of  a  mixture  of  the  arterial  and 
venous  bloods,  and,  consequently,  it  closes  completely 
soon  after  birth.     The  closure  is  made  possible  by  the  fact 
that  during  the  growth  of  the  heart  in  size  the  portion  of 
the  auricular  septum  which  is  between  the  edge  of  the 
foramen  ovale  and  the  dorsal  wall  of  the  auricle  increases 
in  width,  so  that  the  foramen  is  carried  further  and  fur- 
ther away  from  the  dorsal  wall  of  the  auricle  and  comes  to 
be  almost  completely  overlapped  by  the  annulus  of  Vieus- 
sens  (Fig.  131).     This  process  continuing,  the  dorsal  por- 
tion of  the  auricular  septum  finallv  overlaps  the  free  edge 
of  the  annulus,  and  after  birth  the  fusion  of  the  overlap- 
ping surfaces  takes  place  and  the  foramen  is  completely 
closed. 

In  a  large  porcentaKc-  (25  to  30  per  cent.)  of  individuals  the 
fusion  of  the  surfaces  of  the  septum  and  annulus  is  not  complete, 
so  that  a  slit-like  opening  persists  between  the  two  auricles. 
This,  however,  does  not  allow  of  any  mingling  of  the  blood  in 
the  two  cavities,  since  when  the  auricles  contract  the  pressure 
of  the  blood  on  both  sides  will  force  the  overlapping  folds  to- 
gether and  so  practically  close  the  opening.  Occasionally  the 
growth  of  the  dorsal  portion  of  the  septum  is  imperfect  or  is 
inhibited,  in  which  case  closure  of  the  foramen  ovale  is  im- 
possible. 


It      ' 


I. 


THE    ARTERIES. 


261 


^THe  Development  o,  ^;;;^y:^X^^ 
r.:^l':;rs^=nrrt.ns.— .U.e 


X' 


5f^>, 


flUt 


„       .,g Reconstruction  I'h' Embryo  OK  2.6  MM. 

umUucal  vein;  Y.  yolk-stalk.-(//t^.) 

yolk-sac  and  extends  thence  toward  the  embryo.  The 
Zo  oriRinal  omphalomesenteric  artenes.  entering  the 
Tody  of  the  embfyo  along  the  yolk-  stalk,  make  the.  way 
to  the  dorsal  wall  of  the  abdomen,  and  growmg  forward 


w^M\ 


I  I 


262 


TMK    DEVELOPMENT   OF    THE    HIMAN    BODY. 


!  i 


.      1 

hi ! 


V  1* 


//s/ 


and  backward  pive  rise  to  two  longitudinal  stems,  the 
representatives  of  the  dorsal  aorta.  From  near  the  pos- 
terior ends  of  these  there  arise  at  an  early  stage  two 
branches,  which  pass  out  along  with  the  allantois  into  the 
belly-stalk  and  so  to  the  chorionic  villi,  forming  the  alhn- 
toidean  or  umbilical  arteries,  while  anteriorly  each  aorta 

sends  branches  ventrally 
in  the  anterior  branchial 
arches  and  these,  uniting 
together,  pass  backward 
along  the  floor  of  the 
pharynx  to  become  con- 
tinuous with  the  aortic 
bulb  (Fig.  136).  Later 
the  two  dorsal  aortae 
fuse  together  as  far  for- 
ward as  the  region  of  the 
eighth  cervical  segment 
to  form  a  single  trunk 
(Fig.  137),  and  the  left 
omphalo-mesenteric  ar- 
tery disappears,  the  right 
one  persisting  to  form 
the  superior^mesenterr 
arter\\Qf  the  adult. 

It  will  be  convenient 
to  consider  first  the  his- 
tory of  the  vessels  which 
pass  ventrally  in  the  branchial  arches.  Altogether,  sis. 
of  these  vessels  are  developed,  the  f^^urth  branchial 
arch  possessing  a  rudimentary  one  in  addition  to 
that  which  properly  belongs  to  it  (Zimmcrmann),  and 
when  fully  formed  they  have  an  arrangement  which  may 
be  understood  from  the  diagram  (Fig.  137),  in  which  the 


Fh,.  1.^7.  -DiAGKAM  li  I.rSTRATINC, 
THE  AkkAM.EMKnT  ok  HE  HkAN- 
CHIAL  \'ESSEUS. 

(lb,  Aortic  hulh;  da,  di >rsal  aorta:  /  to 
17,  brand! iul  aicli  vessels. 


SBflH 


THE   ARTERIES. 


2^'3 


..nted  as  spread  out  upon  a  plane  surface, 
vessels  are  represented  as  ^P^^*^**  '  .pi^^^  arraiiRe- 

U.e  lateral  trunks  bein,  the  dors^^aot.^^^^^^^  .^  ^^^^^ 

,„ent  represents  a  ^^l^fists  the  respiration  is  per- 
lovver  vertebrates.     I"  r  .,  the  branchial 

formed  by  means  of  K|"^  '^"''  .nhich  receives  venous 
arches,  and  the  heart  .s  an  or.a    ^h  ch  r        ,^^  ^^^  .^^^  .^ 

Hood  from  the  body  ^"^  P^'^^;^  ^  ccf  into  the  dorsal 
becomes  arteriahzed  and  .s  t  In    oU  .^^  ^^^^^^^^.^^ 

aorta.,  which  distribute  It  o  ^j  >"  >  "  .  .^...u.pment  of 
,„Unals,  with  the  loss  of  the  ^^^^^,^,  J  u.e  gills 
the  lungs  as  respiratory  orgu       tla  ^  aP»  ^^^^^^^ 

disappear  and  the  -^^^^^'^,.,.,.,  in  the  dia- 
become  continuous,  the  condition  i.p 

(rram  resulting.  f„,r,norarv  in  the  mamma- 

But  this  condition  .s  merely  '"^^^^'„,,„,  „t  the 

,ia  and  numerous  ehanges  -«""    -/"^^  f,  fi„t  change 

vessels  before  the  ad"H  P'-  -  ^  ;    '^,,1  arch,  the  ven- 
ts a  disappearance  of  the  vesse^  of  the      _^^^_^  ^^^^^^^  ^„ 

tral  stem  from  "f  ^  ;.^;°:;^^;  „^  „ear  the  point  where 
form  the  temporal  ^■^''"f^' «  "  f  ^  branch  which  rep- 
the  branchial  vessel  ong  nally  ^™^  ^  ^^,^    ^,,0  a 

resents  the  internal  ■"»'""''^>';X  fac^lHis).  A  little 
second  branch  which  '^'P^.'^^"  ^J  ' Jf^'l'^enerates  (Fig. 
later  the  second  branclual  «^^el  ^K"  "^  «  „^^,  Hs 

,,,S),  a  branch  -«^ ';:"  '^..ffr^uture  lingual 
former  origm  possibly  "P"^'^^"""^  ,  ,  dor,al  trunk; 
artery  (His),  and  then  '^e  port-n  o^  the  dor     ^_^^^.^^ 

\   which  intervenes  b^t-en  the  tted  --^J°  ^„  ,,, 

\  vessels  vanishes,  so  that  the  ^°^^   '"^„„^^i„„  „uh  the 
third  branchial  arch  IS  cut  off  rom  Its  con 

dorsal  aorta  and  forms  together  wU     tl  e  ves^    ^^^_^^ 

third  arch,  the  internal  carot.d,  "■"';'";;„  ^^,^,_  „,. 

anterior  to  the  point  °f  °"|»  °  ,  ";„\'',tn  which  in- 
comes the  external  carotid,  and  the  por 


264 


TIIK    DIA  KI.DI'MKNT   ol'     TIIK    III  MAN    liOHV 


!■■' 

jiii 
I 

III 


ii 


tervcncs  hetvvet'ii  tlio  third  and  fourth  vissils  htcomes 
the  common  carotid  (Imk-  'V^)- 

The  rudimentary  fifth  vessel,  like  the  first  and  second, 
disappears,  but  the  fourth  persists  to  form  the  aortic  arch, 
there  bein^  at  this  staijc  of  development  two  complete 
aortic  arches.  From  the  sixth  vessel  a  branch  arises 
which  passes  backward  to  the  lunijs.  and  the  portion  of 
the  vessel  of  the  riijht  side  which  intervenes  between  this 


\ 


Fi<;.  l.^S. — Akteriau  Svsti:m  of  an  Iimbryo  ok  10  mm. 

/f,  Internal  carotid;   7',  ])ulm()nary  artery ;    \'r,  vertebral  artery;   ///  to 

\'I,  i>ersistent  branchial  vessels. — (Uis.) 


and  the  aortic  arch  disappears,  while  the  correspondinjj 
portion  of  the  left  side  persists  until  after  birth,  forming 
the  ductus  arteriosus  (ductus  Botalli)  (Fig.  139).  When 
the  longitudinal  division  of  the  aortic  bulb  occurs,  the 
septum  is  so  arranged  as  to  place  the  sixth  arch  in  commu- 
nication with  the  right  ventricle  and  the  remaining  vessels 
in  connection  with  the  left  ventricle,  the  only  direct  com- 
munication between  the  systemic  and  pulmonary  vessels 


•  •-  '^   ^ 


£U 


TIIK     \K  IKK  IKS. 


:••'; 


SI 

e 

X 

i 


hrfon- 


mt^ 


heiug  bywavof  the  ductus  artt-riosus.  whose  siKnit.cantr 

will  be  explaiiKil  later  (p.  2.;n). 

One  other  change  i'^   still    iieeessarv 

scls    acquire    the    ar- 

rauKcnitnt  which  they 

possess    durinu    fetal 

life,  and  thi-    consists 

in    the   disappt       mce 

of    the   lower   portion 

of  the  right  aorf'c  arch 

(Fig.  1.^9),  >o  that  the 

kit  arch  alone  lorms 

the      connection     be- 
tween   the   heart  and 
the  dorsal  aorta.    The 
upper  part  of  the  right 
aortic  arch  persists  to 
form     the     proximal 
part  of  the  right    ub- 
clavian     artery,      the 
portion  of  the  ventral 
trunk  which  unites  the 
arch  with    the    aortic 
bulb     becoming     the 
brachiocephalic     (in- 
nominate) artciy. 

Fron%,  the  entire 
length  of  the  thoracic 
aorta,  and  in  the 
embryo  from  the  aor- 
tic     arches,      lateral 


I'K;     13*1        DiAl'.RAM     luH'STRATING     THK 
(.HANt.ES  IN-  THE  AkRAMIEMENTOF  THK 

liKANCiiiAU  Arch  Vessels. 
The  l)roken  lines  indicate  portions  of  the 

oiiKinul  vesstis  which  have  disappeared. 

A,  Aorta;  AA,  aortic  arch;  Z; .4,  ductus 

artciiosus;    K(",    external   carotid;  IC, 

internal  carotid ;  /.\/,  internal  maxillary ; 

/.    lingual,  /',  pulmonary  artery;  I'A, 
'    pulmonary  aorta;  SA,  systemic  aorta; 

.SV ,  subclavian ;  /  to  I/,  original  branchial 

arch  vessels. 


branches  arise  corresponding  to  each  segment  and  ac- 
companying the  segmental  nerves.  The  first  of  these 
branches  arises  just  below  the  point  of  union  of  the  ves- 


If 

i 

ill 


i 


ji  > 


J  .i.i 


.1 


266 


THE    DEVELOPMENT    OF   THE    HUMAN    BODY. 


S^[ 


sel  of  the  sixth  arch  with  the  dorsal  trunk  and  accom- 
panies the  hypojjlossal  nerve  (Fig.  140,  li),  and  that 
whicli  accompanies  the  seventh  cervical  nerve  arises  just 

above  the  point  of  union 
of  the  two  aortic  arches 
(F'ig.  140,  s),  and  extends 
out  into  the  limb  bud, 
forming  th.':'  subclavian 
artery.* 

Further   down   twelve 
pairs  of  lateral  branches, 
arising  from  the  thoracic 
portion    of     the     aorta, 
represent  the  intercostal 
arteries,  and    still  lower 
four  pairs  of  lumbar  ar- 
teries   are    formed,    the 
fifth  lumbars  being  rep- 
resented   by    two   large 
branches,    the    common 
iliacs,  which   seem  from 
their  size  to  be  the  con- 
tinuations of   the  aorta 
rather  than  branches  of 
it.     The  true   continua- 
tion of  the  aorta  is,  how- 
ever, the  art,  saQra  media, 
which  represents  in  a  de- 
generated form  the  caudal  prolongation  of  the  aorta  of 
other  mammals,  and,  like  this,  gives  off  late.al  branches 
corresponding  to  the  sacral  segments. 

*  It  must  lie  remembered  that  tlie  right  subclavian  of  the  adult  is  more 
than  equivalent  to  the  left,  since  it  represents  the  fourth  branchial  vessel 
f  a  portion  of  the  dorsu!  Umgitudinal  trunk  +  the  lateral  segmental 
branch  (see  Fig.  140). 


Tk;,  140.  —  Dl.AGR.\M  SHOWINT,  THE  Re- 
l.\TIO.VS  OF  THE  L.ATER.M,  BRANCHES 

TO  THE  .\oRTic  Arches. 
EC,  External  carotid ;  //,  lateral  branch 
accompanying  the  hypoglossal  nerve ; 
IC,  internal  carotid ;  ICo,  intercostal ; 
IM,  internal  mammary ;  *,  sub- 
clavian; r,  vertebral;  /  to  l'///, 
lateral  cervical  branches;  1,  2, 
lateral  thoracic  branches. 


>31B 


THE    ARTERIES. 


267 


In  addition  to  the  segm™tan2lSaLUmi£liei-ai:^ 
from  the  aorta,  viscmLbrand..:=..-^MclU---*''«r  ongm 
rather  frcn  UiSL^ailniLsurlaciabalUte  suU-s,  also  occu 
The  de";^eTopment  of  these  branehes  has  as  yet  been  but 
U  t  e  studied,  but  it  seems  probable  that  they  too  may 
show  when  their  embryonie  history  has  been  worked  out, 
a  more  perfect  segmental  plan  than  is  diseenuble  from 
their  adult  arrangement.      The 
earliest  representative  of  them  is 
the  superior  mesenteric  artery, 
whose  origin  from  the*^t  m\]^ 
nhalo-mesentjLric     has     already 
been  described.      Several   other 
visceral  branches  occur  both  in 
the  thoracic  and  abdominal  re- 
gions, but  they  are  irregular  in 
their  distribution,  the  unpaired 
branches  of   the  abdominal  re- 
gion having  piobably  condensed 
from  an  original  segmental  con- 
dition to  form  compound  trunks 
such  as  the  coeliac  axis  and  the 
inferior  mesenteric. 

One  pair  of  the  visceral 
branches,  the  umbilical  arteries, 
require  more  than  a  passing 
notice  on  account  of  their  en.- 

bryonic  importance.  They  are  formed  at  a  very  early 
stage  and  arise  by  a  short  common  trunk  from  the  anterior 
surface  of  the  aorta  (Fig.  141,  ^M-  They  pass  directly 
forward  on  the  medial  side  of  the  Wolffian  duct  (see  p.  36 1 ) 
to  the  terminal  portion  of  the  intestine,  and  thence  p^-'S 
out  along  the  sides  of  the  allantois  to  the  chorionic  villi. 
Later  there  are  formed  from  ti..     orta  just  below  the  origin 


Fit 


>,     141. — Diagram    Iturs- 

TRATINO  THE  DEVELOPMENT 

OF  THE  Umbilical  Arter- 
ies. 
■I,  .\()rta;  CI  I,  coninum  iliac; 
Ell,  external  iliac;  ///,  in- 
ternal iliac;  T,  umbilical 
artery;  l'\  the  i)riniary 
proximal  and  ^'^the  secon- 
dary proximal  part  of  the 
umbilical  ;  wd,  Wolffian 
duct. 


';U' 


li   ! 


iMi 


t  f' 


■  I 
I  ( 


I  I 


If 
ft 

Ml 
Is' 


268  THE    DE    ELOPMENT   OF    THE    HUMAN    HOOY. 

of  the  umbilicals.  the  lateral  branches  (c.il)  which  be- 
come the  common  iliacs,  and  from  each  of  these  a  short 
branch  (V)  arises  which  passes  to  the  outer  side  of  the 
Wolffian  duct  and  unites  with  the  umbilical  arteries, 
whereupon  the  original  proximal  portions  of  these  arteries 
disappear  and  they  come  to  arise  from  the  iliacs  instead  of 
directly  from  the  aorta.    At  birth  the  portions  of  the  arte- 
ries beyond  the  umbilicus  are  severed  when  the  umbilical 
cord  is  cut,  and  their  intra-embryonic  portions,  which 
have  been  called  the  hypogastric  arteries,  quickly  undergo 
a  reduction  in  size.     The  proximal  portion  of  the  allantois 
persists  as  the  urinary  bladder,  and  the  proximal  portions 
of  the  hvpogastric  arteries  remain  functional  as  the  supe- 
rior vesical  arteries  carrying  blood  to  this  viscus,  but  the 
portions  which  intervene  between  the  bladder  and  the 
umbilicus  become  reduced  to  solid  cords  forming  the  ob- 
literated hvpogastric  arteries  of  adult  anatomy. 

In  its  general  plan,  accordingly,  the  arterial  system  may 
be  tegarded  as  consisting  of  a  pair  of  longitudinal  vessels 
whi.>h  fuse  together  throughout  the  greater  portion  of 
their  length  to  f-.rm  the  dorsal  aorta,  from  which  there 
arise  lateral,  segmentally  arranged  somatic  branches  and 
ventral  visceral  branches  whose  segmental  arrangement 
is  less  distinct.     With  the  exception  of  the  aortic  trunks 
(together  with  their  anterior  continuation,  the  internal 
carotids)  and  the  external  carotids,  no  longitudinal  arte- 
ries exist  primarily.     In  the  adult ,  however,  several  longi- 
tudinal vessels,  such  as  the  vertebrals,  internal  mammary, 
and  epigastric  arteries,  exist.     The  formation  of  these 
secondary  longitudinal  trunks  is  the  result  of  a  develop- 
ment between  adjacent  vessels  of  anastomoses,  which 
become  larger  and  more  important  blood -channels  than 
the  original  vessels. 

At  an  early  stage  each  of  the  lateral  branches  of  the  dor- 


THE    ARTERIES. 


269 


sal  aorta  gives  off  a  twis  which  passes  forward  to  anas- 
tl^  wfth  a  backwardly  directed  tw.g  from  the  next 


A.V.CT. 


Vu:     .42.      TMK   1.KVKU...MKNT  OK  THK  VEKTKH^U  AkTKKV   .N    X    K.UB.T 

"   •  Fmbryo  OF  IwELVK  Dass. 

/.S/..6\' spinal  gan-lion.     {Ilochstctlcr.) 

anterior  lateral  branch,  so  as  to  form  a  longitudinal  drain 
of  anastomoses  alon,  each  side  of  the  neck^   In  the  earhes 
stage  at  present  known  the  chain  starts  from  the  lateral 


270 


THK    DEVELOPMENT    OF     THE    HLMAN    BODY. 


branch  corresponding  to  the  first  cervical  ("suboccipital) 
segment  and  extends  forward  into  the  skull  through  the 
foramen  magnum,  terminating  by  anastomosing  with  the 
internal  carotid.  To  this  original  chain  other  links  are 
added  from  each  of  the  succeeding  cervical  lateral 
branches  as  far  back  as  the  seventh  (Figs.  142  and  140). 
But  in  the  mean  time  the  recession  of  the  heart  toward  the 


":■  i 


! 


¥ 


S« 


m 

li 


ii    , 


■i 


Fir,     14.^       I{mbkvo  'H     1^    mm     miowinc  the   Mode  oV    Deveuoi-mENT 
OF   THE    IntEknai     Mammary   am.    Deei'    Iu-k.astkic   Arteries. — 

(A/,j//,)' 

thorax  has  begun,  with  the  result  tliat  tlie  common  carotid 
stems  are  elongated  and  the  aortic  arches  are  apparently 
shortened  so  that  the  subclavian  arises  on  the  left  side 
almost  opposite  the  point  where  the  aorta  was  joined  by 
the  sixth  branchial  vessel.  As  this  apparent  shortening 
proceeds,  the  various  lateral  branches  which  give  rise  to 
the   chain  of   anastomoses,  with   the   exception   of   the 


THE    ARTERIES. 


271 


seventh,  disappear  in  their  proximal  portions  and  the 
chain  b  come"  an  independent  stem,  the  .ertetra  artery, 
arising  from  the  seventh  lateral  branch,  which  is  the  sub- 

""^^ThTrecession  of  the  heart  is  continued  until  it  lies  below 
the  level  of  the  upper  intercostal  arteries  and  the  upper 
two  of  these,  together  with  the  last  cervica  branch  on 
each  side  lose  their  connection  with  the  dorsal  aorta,  and, 
sending  off  anteriorly  and  posterioriy  anastomosing  twigs 
develop  a  short  longitudinal  stem,  the  supenor  tntercostal, 
which  opens  into  the  subclavian. 

The  intercostals  and  their  abdominal  representatives, 
the  lumbars  .«d  iliacs,  also  give  rise  to  longitudinal  anas- 
tomosing twigs  near  their  ventral  ends  (Fig.  143).  and 
these  increasing  in  size  give  rise  to  the  internal  mammary 
and  deep  epiqastric  arteries,  which  together  form  con- 
tinuous stems  extending  from  the  subclavians  to  the  exter- 
nal iliac,  in  the  ventral  abdominal  walls.  The  superficial 
epigastrks  and  other  second^longitudinal  vessels  are 

formed  in  a  similar  manner  .^r^     x  -^       \,'    u 

The  Development  of  the-  ArteVies  of  the  Limbs.-Much 
information  is  still  required  before  the  complete  history 
of  the  development  of  the  arteries  of  the  limbs  can  be 
written,  and  at  present  one  must  rely  largely  upon  the 
facts  of  comparative  anatomy  and  on  the  anomalies  which 
occur  in  the  human  body  for  indications  of  what  the  eariy 
development  is  likely  to  be.  So  far  as  embryological  ob- 
servations go,  they  confirm  the  conclusions  derived  from 

such  sources. 

Notwithstanding  the  fact  that  the  limbs  are  formed  by 
outgrowths  from  several  segments,  there  is  as  yet  no  evi- 
dence to  show  that  a  corresponding  number  of  segmental 
arteries  take  part  in  the  development  of  their  blood-sup- 
ply, but  it  seems  that  in  both  limbs  the  entire  arterial  sys- 


) 


272 


THE    DEVELOPMKNT    OF    THE    HUMAN    BODY. 


tern  is  formed  from  a  single  lateral  branch,  that  of  the 
upper  limb,  the  subclavian,  corresponding  to  the  seventh 
cervical  segment,  while  that  of  the  lower  limb,  the  com- 
mon iliac,  is  probably  the  fifth  lumbar  branch.  In  the 
simplest  arrangement  the  subclavian  is  continued  as  a 
single  trunk  along  the  axis  of  the  anterior  limb  as  far  as 
the  carpus,  where  it  divides  into  digital  branches  for  the 
fingers.  In  its  course  through  the  forearm  it  lies  in  the 
interval  between  the  radius  and  ulna,  resting  on  the  inter- 
osseous membrane,  and  in  this  part  of  its  course  it  may  be 
termed  the  arteria  interossea.  In  the  second  stage  a  new 
artery  accompanying  the  median  nerve  appears,  arising 
from  the  main  stem  or  brachial  artery  a  little  below  the 
elbow-joint.  This  may  be  termed  the  arteria  vicdiana, 
and  as  it  develops  the  arteria  interossea  gradually  dimin- 
ishes in  size.  l>ccoming  fmallv  the  small  anterior  interosse- 
ous artery  of  the  adult  (Fig.  144),  and  the  median,  uniting 
with  its  lower  end.  takes  from  it  the  digital  branches  and 
becomes  the  principal  stem  of  the  forearm. 

A  third  stage  is  then  ushered  in  by  the  appearance  of  a 
branch  from  the  median  w'nich  forms  the  arteria  ulnaris, 
and  this,  passing  down  the  ulnar  side  of  the  forearm, 
unites  at  the  wrist  with  the  median  to  form  a  superficial 
palmar  arch  from  wliich  the  digital  branches  arise.     A 
fourth  stage  is  marked  by  the  diminution  of  the  median 
artery  until  it  finally  appears  to  be  a  small  branch  of  tlie 
interosseous,  the  arteria  comes  ncrvi  mediant,  and  at  the 
same  time  tlure  develops  from  the  brachial,  at  about  the 
middle  of  the  upper  arm,  what  is  knov.n  as  the  arteria 
radialis   supcrficialis     Fig.  144,  'V.     This  extends  down 
the  radial  side  of  the  forearm,  following  the  course  of  the 
radial  nerve,  and  at  the  wrist  passes  upon  the  dorsal  sur- 
face of  the  hand  to  form  the  na.  dorsalis  pollicis  and  dor- 
salis  india.\.     At  first  tlii ,  artery  takes  no  part  in  the  for- 


I    ' 


THE    AKTKKIES. 


273 


m 


i 


mation  of  the  palmar  arches,  but  later  it  gives  rise  to    he 

runerfic^al  volar  branch,  which  usually  unites  with  the 

Xrficial  arch,  while  from  its  dorsal  portion  a  perforating 

branch  develops  which  passes  between  the  first  and  second 


,.•„..      144        D.A..RAMS    SHOWINU    AN    EaRLV    AN,,    A    I.^TE    S-Ar-E    IN     THE 
DEVELOPMENT   OK    THE    ARTERIES    OK    THE    ArM. 

/,   Hracl.ial ;  i.  interosseous;  m,  comes  neryi  n,ediani ; ,,  radial ;  rv,  supertieial 

radial;  u,  ulnar. 

metacarpal  bones  and  unites  with  a  deep  branch  of  the 
ulnar  to  form  the  deep  arch.  The  fifth  or  adult  stage  is 
reached  by  the  development  from  the  brachial  below  the 
elbow  of  a  branch  (Fig.  144,  ')  which  passes  downward 
and  outward  to  unite  with  the  superficial  radial,  whcre- 


274 


THE    DEVELOPMENT    OF    THE    HUMAN    KODY. 


k 


h\ 


m 


upon  the  upper  portion  of  that  artery  degenerates  until  it 
is  represented  only  by  a  branch  to  the  biceps  muscle 
(Schwalbe),  while  the  lower  portion  persists  as  the  adult 
radial. 

The  various  anomalies  seen  in  the  arteries  of  the  forearm  are, 
as  a  rule,  due  to  the  more  or  less  complete  persistence  of  one  or 
other  of  the  stages  described  above,  what  is  described,  for  m- 
stance,  as  the  high  branching  of  the  brachial  being  the  per- 
sistence of  the  superficial  radial. 

In  the  leg  there  is  a  noticeable  difference  in  the  arrange- 
ment of  the  arteries  from  what  occurs  in  the  arm,  in  that 
the  principal  artery  of  the  thigh,  the  femoral,  does  not 
accompany  the  principal  nerve,  the  sciatic.     This  differ- 
ence is  apparently  secondary,  but,  as  in  the  case  of  the 
upper  limb,  it  is  necessary  to  rely  largely  on  the  facts  of 
comparative  anatomy  and  on  anomalies  which  occur  in 
the  human  body  for  an  idea  of  the  probable  development 
of  the  arteries  of  the  lower  limb.     It  has  already  been  seen 
that  the  common  iliac  artery  is  to  be  regarded  as  a  lateral 
branch  of  the  dorsal  aorta,  and  in  the  simplest  condition 
of  the  limb  arteries  its  continuation,  the  anterior  division 
of  the  internal  iliac,  passes  down  the  leg  as  a  well-devel- 
oped sciatic  artery  as  far  as  the  ankle  (Fig.  HS,  -^^^     At 
the  knee  it  occupies  the  position  of  the  popliteal  of  adult 
anatomy,  and  below  the  knee  gives  off  a  brancli  corre- 
sponding to   the  anterior  tibial  (at)  which,  passing  for- 
ward to  the  extensor  surface  of  the  leg,  quickly  loses  itself 
in  the  extensor  muscles.     The   main   artery   continues 
downward  on  the  interosseous  membrane,  and  some  dis- 
tance above  the  ankle  divides  into  a  strong  anterior  and  a 
weaker  posterior  branch;  the  former  perforates  the  mem- 
brane and  is  continued  down  tlie  extensor  surface  of  the 
leg  to  form  the  lower  part  of  the  anterior  tibial  and  the 
dorsalis  pedis  arteries,  while  the  latter,  passing  upon  the 


111 


TIIK    AKir.KIK.S. 


275 


plantar  surface  of  the  foot,  is  lost  in  the  plantar  muscles. 
At  this  stage  the  external  iliac  is  a  secondary  branch  of  the 
common  iliac,  being  but  poorly  developed  and  not  extend- 
ing as  far  as  the  knee. 

In  the  second  stage  the  external  iliac  artery  increases  in 
size  until  it  equals  the  sciatic,  and  it  now  penetrates  the 


6a 


S 


rr\ 


at 


f\ 


X 


u^ 


V\r,.    145. 


Dia(;k\ms  Illistrati.m;  St.vcves  in  thb  Devbuoi'ment  of 
THi:  Arteries  ok  the  Lei,. 
.ri,  Anterior  tibial;  .//>,  dnrsalis  ;)cais;  /,  feniural;  p,  popliteal;  pc,  poront-al; 
pi,  poMtTior  tibial;  f,  sciatic;  sn,  saphenous. 

adductor  magnus  muscle  and  unites  with  the  popliteal 
portion  of  the  sciatic.  Before  doing  this,  however,  it  gives 
off  a  strong  branch  isa)  which  accompanies  the  long  saph- 
nous  nerve  down  the  inner  side  of  the  leg,  and  passing  be- 
iMnd  the  internal  malleolus  extends  upon  \\,-c  plantar  sur 


270 


THE    DKVKI.OI'Ml.NT    OF     1111.    IIIMAN    lloDV. 


U   i 


tl ; ; 


i  i 


I; 


face  of  the  foot,  where  it  gives  rise  to  the  digital  branches. 
From  this  arrangement  the  adult  condition  niav  be  de 
rived  by  the  continued  increase  in  size  of  the  external 
iliac  and  its  contiiuiation,  the  femoral  (/),  accompanied 
by  a  reduction  of  the  up|K  r  portion  of  the  s'  iatic  and  its 
separation  from  its  popliteal  portion  (p).  The  continua- 
tion of  the  popliteal  down  the  leg  is  the  peroneal  artery 
(/)<•)  and  the  upper  perforating  branch  of  this  unites  with 
the  lower  (me  to  form  a  continuous  anterior  tibial,  the 
lower  connection  of  which  with  the  peroneal  ])ersists  in 
])art  as  the  anterior  peroneal  artery.  A  new  branch 
arises  from  the  upper  part  of  the  peroneal  and  passes  down 
llie  back  of  the  leg  to  unite  with  the  lower  part  of  the 
(iilcria  sdphcna,  forming  the  posterior  tibial  artery  (pi), 
and  the  upper  part  of  tlie  saj)henous  becomes  much  re- 
duced, persisting  as  the  superhcial  branch  of  the  anas- 
tomotica  nia^^na  (artcria  musculo-a)ticuli'ris)  and  a  rudi- 
mentary chain  of  anastomoses  wliieh  accompany  the  long 
saphenous  nerve. 

The  Development  of  the  Venous  System. ^The  earliest 
veins  to  develop  are  those  which  accompany  the  first - 
formed  arteries,  the  omphalo-mcsenterics  and  umbilicals, 
but  it  will  be  more  convenient  to  consider  first  the  veins 
which  carry  the  blood  from  the  body  of  the  embryo  back 
to  the  heart.  These  make  their  appearance,  while  the 
heart  is  still  in  the  pharyngeal  region,  as  two  pairs  of  longi- 
tudinal trunks,  the  anterior  and  posterior  cardinal  veins, 
into  wliich  lateral  branches,  arranged  more  or  less  seg- 
mentally,  open.  The  anterior  cardinals  appear  somewhat 
earlier  than  the  posterior  and  form  the  internal  jugular 
veins  of  adult  anatom> .  They  are  formed  by  the  union 
of  two  stems  which  convey  tlie  blood  from  the  brain.  One 
of  these  stems  is  formed  by  a  number  of  veins  which  pass 
backward  over  the  surface  of  the  fore-brain,  uniting  to 


aa 


TllK    VKINS. 


2-n 


I 


f..rm  a  stem  which  follows  the  course  of  the  facial  nerve 

«-1,icl.  thoy"for.M«l  becomes  the  hleml  s,„us  (sir,  «.t  . 

id  I  .■  oph.  halnnc  voin  (»..  1  unit.-s.   A  con.mu.nca.u, 

I"  n,  on  tlu.  .ncdi..,  si.K.  of  .l,c  car  capsule,  dcvclo  s 

icl     c"  11.C  luural  sinus  ami  ll.c  ,>oslcrior  slcn,  an.l  the 

i'  :,:;:!:nn,unication  disappears,  ..,c  -;•;;.>■■-.- 

„s  ,l,c  forc-l-rain  vesicles  srow    '-•''"■^■"';;  "    'T'  ' 
„„i„„  „t  .l.e  superior  lonsiludinal  sn.us  .,1.  ^'^^ 
is  brouKl.t  nearer  to  its  adult  position,  and    at  t  R  same 
,       tl,e  portion  of  the  lateral  sinus  between  the  con,- 
;;;,nication  of  the  ophthalmic  vein  -';.''-,;;^,;'-   ^  .:• 
,„.n  dinrinishes  in  length  until  .t  .s  P"-""^'"'""'^  „'"';'; 
the  ophthuhnics  and  lateral  sinuses  .neetmK  at  the  JURular 
forZ-n.     The  intra-crunial  portions  of  the  ophthahn.c 
c  ns  form  the  cveruous  and  n.lerior  pdros.l  snmses. 
U,l  superior  petrosals  being  formed  later  by  a  con.mumca- 
tion  between  the  cavernous  and  lateral  sinuses 

Passing  backward  from  the  jugular  foramen  the  mternal 
juKular  4ins  unite  with  the  inferior  cardinals  to  form  on 
each  side  a  common  trunk,  the  duclus  C«ur,,  and  then 
passing  transversely  toward  the  median  line  open  into  the 
!^,es  of  the  sinus  venosus.    So  long  as  the  heart  retains  Us 
original  position  in  the  pharyngeal  region  the  jugular  is  a 
short  trunk  receiving  lateral  veins  only  from  the  upper- 
most  segments  of  the  neck  and  from  the  occipital  seg- 
ments, the  remaining  segmental  veins  opening  into  the 
inferior  cardinals.     As  the  heart  recedes,  howev".  the 
jugulars  become  more  and  more  elongated  and  the  cervi- 


1.0    !i: 


I.I 


1.25 


■  50 

illlM 

m, 

1^6 

1^ 

ill  ^'^ 

1^ 

i^ 

us. 

b 

%im 

i^ 

1.4 


1.8 


1.6 


MICROCOPY  RESOLUTION  TEST  CHART 

NATIONAL  BUREAU  OF  STANDARDS 

STANDARD  REFERENCE  MATERIAL  1010a 

(ANSI  and  ISO  TEST  CHART  No.  2) 


,:'       t 


^       i 


i    1 


SI        > 


Fig.   146. — Rkcon'sthuction  of  thk  Head  Veins  of  GriNEA-i-u;  Em- 
bryos. 
.4,  Hyi-;  d,  audilmy  cajjsuk-;  >.u--,  superior  longitudinal  siuu;;;  i/^,  lateral 
sinus;  vja,  facial  vein;  vj  and  vji,  internal  jugular  vein;  vje,  external 
jugular  vein;  w,  ophthalmic  vein. — {Salzcr.) 

278 


^mt 


TllK    VKINS. 


-/V 


I 

i 


cardinals^     \\  litn  tut  mhu  Cuvicrian 


across  to  join  the  risht  jugular,  formiug  the  left  .W«- 
Zle.eJ  When  '^s  i.  estahl.sla.l  the  eo„r,ect^^  be- 
tween the  left  jugular  and  Cuv.cnan  duct  -  'I'^-l-^;  "^^ 
blood  from  the  left  side  of  the  head  and  neck  and  from  Uw. 
left  subclavian  vein  passing  over  to  ™M>  ^ ;"*°.  '^Vvt 
jugular,  whose  lower  end,  together  w>th  the  "SW  tuMc 

an  duct,  thus  becomes  the  super, or ^n^  ,T,,  Jof  t 
Cuvierian  duct  persists,  forming  w>th  the  left  horn  of 

sinus  venosus  the  coronary  sinus 


ir 


\ 


i  i^ 


280 


THE    nF.VF.I.Ol'MKNT    OK   THK    HUMAN 


HO  in 


The  external  jugular  vein  develops  somewhat  later  than 
the  internal.     The  facial  vein,  which  primarily  forms  the 
principal  affluent  of  this  stem,  passes  at  first  into  the  skull 
along  with  the  fifth  nerve  and  communicates  with  the  m- 
ternal  jugular  system,  but  later  this  original  communica- 
tion is  broken  and  the  facial  vein,  uniting  with  other 
superricial  veins,  passes  over  the  jaw  and  extends  down 
the  neck  as  the  external  jugular  (Fig.  146,  ^/t')-     Later 
still  the  facial  anastomoses  with  the  ophthalmic  at  the 
inner  angle  of  the  eye  and  also  makes  connections  with  the 
internal  jugular  just  after  it  has  crossed  the  jaw,  and  so 
the  adult  condition  is  acquired. 

It  is  interesting  to  note  that  in  nianv  of  the  lower  mammals 
the  external  jugular  becomes  of  much  greater  importance  than 
the  internal,  the  latter  in  some  forms,  indeed,  eventually  disap- 
pearing and  the  blood  from  the  interior  of  the  skull  emptying  by 
means  of  anastomoses  which  have  developed  into  the  external 
iugular  svstem.  In  man  the  primitive  condition  is  retamed,  > 
but  indications  of  a  transference  of  the  intracranial  blood  to  the 
external  jugular  are  seen  in  the  emissary  leins. 

The  inferior  cardinal  veins,  or,  as  they  may  more  simply 
be  termed,  the  cardinals,  extend  backward  from  their 
union  witli  the  jugulars  along  the  sides  of  the  vertebral 
column,  receiving  veins  from  the  mesentery  and  also  the 
various  lateral  segmental  veins  of  the  neck  and  trunk 
regions,  with  the  exception  of  that  of  the  first  cervical 
segment  which  opens  into  the  jugular.     Later,  however, 
as'klreadv  described  (p.  279),  the  cervical  veins  shift  to  the 
jugulars,' as  do  also  the  first  and  second  thoracic  (inter- 
costal) veins,  but  the  remaining  intercostals,  together  with 
the  lumbars  and  sacrals,  continue  to  open  into  the  cardi- 
nals.    In  addition,  the  cardinals  receive  in  early  stages  the 
veins  from  the  primitive  kidneys  (mesonephros),  which 
are  exceptionally  large  in  the  human  embryo,  but  when 
they  are  replaced  later  on  by  the  permanent  kidneys 


THE    VEINS. 


2S1 


(metanephros)  their  vc-h.s  u.uUtro  a  r.ch.ct.on  u.  n"n.h.r 
a"  a,„l    his,  tcscthcT  with  the  shifting  of  the  uppe 
Lteral  veins,  produces  a  marlced  <liminutio„  m  the  sue  o 
the  car  linal  .     These  veins  persist,  however,  m  part  unt.l 
alh  lite  f„r,ning  what  are  known  as  the  „w^  and  /a.,,  - 
"to     r  ..,  i.«t  the  changes  hy  which  they  -q-'^'  "  " 
finaTarrangien,  are  so  i„ti„.ately  ^^^l;;^j;^^_ 
develop,ner,t  of  the  inferior  vena  cava  ''  ''\'     ';  ''^^  "'^f 
tion  may  l)e  conveniently  postponed  until  the  history  ol 
that  vdn,  together  with  that  of  the  oniphalo-nicser.tenc 
■ind  of  the  umhilical  veins,  has  been  presented 

Tte  o,„plu,lo-,,,ese,„eric  reins  are  two  in  number  a  right 
and  a  It,  and  pass  in  along  the  yolk-stalk  until  tney 
r^ach  the  embryonic  intestine,  along  the  sides  of  which 
hey  pass  forwanl  to  unite  with  the  corresponding  MIBbJi. 
ca   veins.     These  are  represented  in  the  bcly-stalk  by  a 
stogie  venouIlT^nk  which,  when  it  .caches  the  body  of  the 
eX-o,  divides  into  two  stems  which  pass  forward  one 
on  each  side  of  the  umbilicus,  and  thence  on  each  side  of 
the  median  line  of  the  ventral  abdominal  wall,  to  form 
with  the  corresponding  omphalo-mesenterie  veins  common 
Trunks  which  open  into  the  ductus  Cuvieri.     As  the  liver 
levdops  it  comes  into  intimate  relation  with  the  omphalo- 
mlcnteric  veins,  which  receive  numerous  branches  from 
Us  substance  and,  indeed,  seem  to  break  up  into  a  network 
(Fig   .48,  A)  traversing  the  liver  substance  and  unitmg 
again  to   orm  two  stems  which  represent  the  onginal  con- 
tinuations of  the  omphalo-niesenterics     Krom  the  po.^ 
here  the  common  trunk  forn.ed  by  the  right  omphalo- 
mesenteric and  umbilical  veins  opens  into  t_  -  Cuv^erian 
duct  a  new  vein  develops,  passing  <'°«"''-  '■•'.^Z'.  '°   '''^ 
left  to  unite  with  the  left  omphalo-mescnteric ,  thl.  is  the 

^T^-      ,  .Q    n    TlVA^      In  the  mean  time 
ductus  venosus  (Fig.  148.  B,  Ui  /i).     a"  ^-"^ 

three  cross-connections  have  developed  between  the  t^^o 


24 


f 


2o2  TliK    DKVKLoi'MKNT    OF    JIIK    HUMAN    IIODV. 

oinplialo-nicscntcric  veins,  two  of  which  pass  ventral  and 
the  other  dorsal  to  the  intestine,  so  that  the  latter  is  sur- 
rounded by  two  venous  loops  (Fig.  149,  A),  and  a  connec- 
tion is  developed  between  each  umbilical  vein  and  the 
corr  sponding  omphalo-mesenteric  (Fig.  148,  B),  that  of 
theleft  side  being  the  larger  and  uniting'  with  the  omphalo- 
mesenteric just  where  it  is  joined  by  the  ductus  venosus 
so  as  to  seem  to  be  the  continuation  of  this  vessel  (Fig.  148, 


J)C. 


■  f 


I 

i 


\  <• 


"^(O?'^ 


DC 


Vomd 


'r\\     ' 


m-yi 


rus 


lOMIi 


J).VA 


i^:/YA 


f^e/s 


Vomd 


148. 


I)r.\<;K.\MS     iLtt  STKATIM,     TIIK     Tk.\.\SF()RM.\TI().\S     OK 
()MrH.\LO-MKsK.\TKKIC  AND   ImBILICNL  \'KINS. 


THE 


D.C,   Dn'?tiis    Cuvicri;     ).\'.A,   ductus   venosus;     I'.o.m.;/    and   V.o.m.!;, 
riKlu    and    left    oniplialo-niescnteric    veins;    V .u.d   and    V .ti.s,    rigiit 


and  left  unil)ilical  vcms.—iH oclisMIci:) 


C).  When  these  connections  are  complete,  the  upper 
portions  of  the  umbilical  veins  degenerate  (Fig.  149),  and 
now  the  right  side  of  the  lower  of  the  two  omphalo-mesen- 
teric loops  which  surround  the  intestine  disappears,  as 
does  also  that  portion  of  the  left  side  of  the  upper  loop 
which  intervenes  between  the  middle  cross-connection  and 
the  ductu  venosus,  and  so  there  is  formed  from  the  om- 
phalo-mesenteric veins  the  vena  porta. 


•'"^^'TSPS'^'-iSW" 


TUR    VEINS. 


:83 


^ 


While  tht'sc-  changes  have  been  progrcssinR  the  right 
u.nbihcal  vcin.originally  the  larger  of  the  two  (l<ig. .  48.  A 
■ind  B  V  u.d),  has  become  very  much  reduced  m  size 
and.  losing  its  connection  with  the  left  vein  at  the  ui-ilnh- 
cus,  forms  a  vein  of  the  ventral  abdominal  wall  m  wluch 
the  blood  now  flows  from  above  downward.  The  Ictt 
utnbilical  now  forms   the  only  route  for  the  return  of 


,4V  \     TilK  \KNO.JS  TKrNKS  r,K  an   IvMHKVO  ok  5  MM.    SHUN    1-KOM 

THE    VENTKA.:    SuKI-ACE;     1^,     D.AGKAM     ItUrSTRATINr.    TUE     1 RAN.- 
l-OKMATION    TO   THE    AdULT   CONDITION. 

Vcd    and    l  '  "  '    '  '^    " ' 


Kk;.  149. 


l-OKMATION    TO   THE    AdULT   CONDITION. 

■>nd  IVs  riL'ht  luul  left  sui>eri.-r  vcnx  c;;v;r;  Ij,  jugular  vein ; 
VZ.  o.u,^.al"-.  .esenteric  vein  V^  vena  porta;  ''.  "j^;^'' l^^ 
vein  (lower  part);  Vu\  umbilical  vein  (upper  par  ) ,  V  «;/ and  V  us, 
right  and  left  umbilical  veins  (lower  parts).— (/iw.) 

blood  from  the  placenta,  and  appears  to  be  the  direct  con- 
tinuation of  the  ductus  venosus  (Fig.  i49,£^.  i"to  which 
open  the  hepatic  veins,  -eturning  the  blood  distributed 
by  the  portal  vein  to  the  substance  of  the  liver. 

Returning  now  to  the  cardinal  veins,  it  has  been  found 
in  the  rabbit  that  the  branches  which  come  to  them  from 
the  mesentery  anastomose  longitudinally  to  form  a  vessel 


I  ? 


284 


THK    DKVEI.OPMENT    OF    THE    HUMAN    IIODY. 


lyinj(  parallel  and  slij^htly  ventral  to  each  cardinal.  These 
may  be  termed  the  suhairdinal  vchis  (Lewis),  and  in  their 
earliest  condition  they  open  at  either  end  into  the  corre- 
sponding cardinal,  with  which  they  are  also  united  by 
numerous  cross-branches.  Later,  in  rabbits  of  8.8  mm., 
these  cross-branches  begin  to  disappear  and  give  place  to 
a  large  cross-branch  situated  immediately  below  the  origin 
of  the  superior  mesenteric  artery,  and  at  the  same  point  a 
cross-branch  fjctween  the  two  subcardinals  also  develops. 
The  portion  of  the  right  subcardinal  vvhich  is  anterior  to 
the  cross-connection  now  rapidly  enlarges  and  unites  with 
the  ductus  venosus  just  where  the  hepatic  veins  open  into 
that  vessel  (Fig.  150,  A),  and  at  the  same  time  the  lower 
part  of  the  right  cardinal  and  its  connection  with  the 
right  subcardinal  also  enlarge,  and  these  three  elements 
straighLci?  out  to  form  a  single  longitudinal  trunk,  which, 
together  with  the  proximal  portion  of  the  ductus  veno- 
sus, constitutes  the  voia  cava  injerior  of  the  adult. 

As  soon  as  the  establishment  of  this  vessel  is  accom- 
plished, the  lower  portion  of  the  right  subcardinal  under- 
goes degeneration,  while  the  left  one,  diminishing  in  size, 
persists  as  the  left  suprarenal  vein  (Hochstetter)  (Fig.  150, 
B,  vsr).  The  cross-branch  between  the  two  subcardinals 
persist'-.,  however,  and  by  its  connection  with  the  left 
cardinal  allows  the  blood  from  the  lower  part  of  that  vein 
to  flow  over  into  tlie  vena  cava. 

As  the  permanent  kidneys  grow  forward  (see  Chap, 
xiii)  they  push  their  way  between  the  aorta  and  the  poste- 
rior portions  of  the  cardinal  veins,  forcing  the  latter  off  to 
the  side  and  interfering  with  the  flow  of  blood  in  them,  a 
difficulty  which  is  overcome  by  the  development  of  a 
branch  from  each  cardinal,  just  above  the  kidney,  which 
passes  to  the  medial  side  of  the  ureter  to  unite  again  with 
the  cardinal  below  (Fig.  150,  B).     As  soon  as  this  circle 


5:^^S~^-i 


THK    VF.INS. 


185 


around  the  ureter  has  been  established,  its  lateral  htnb, 
u  nich  represents  part  of  the  oriKinal  cardinal  vein,  degen- 
erates, its  anterior  portion  alone  persisting  to  form  a  part 
of  the  renal  vein  (compare  Kigs.  150,  A  and  B,  r).  An 
anastomosis  now  deveh)ps  between  the  right  and  left  car- 
dinals at  the  point  where  the  iliac  veins  open  into  them 


irf 


j.-j,;    150.  — Diagrams  Iulkstkatinc.  the  Dkvei.oi'mknt  ok  the  Inferior 

Vena  Cava. 

cs,  C.ronury  sinus;  dv,  ductus  vcnosus;  H,  iliuc  vein;  r,  reaal;  scl  sul)- 
clavian;  sp,  spermatic;  va,  vena  a/.v^os;  vh,  hepatic;  vIm,  vena  lienu- 
azysos;  ri,  left  innominate;  r/,  jugular;  rvc,  sul)car(linal;  vsr,  supra- 
renal. 

(Fig.  150,  B),  and  the  portion  of  the  left  cardinal  whi  h 
intervenes  between  this  anastomosis  and  the  entrance  of 
the  spermatic  (ovarian)  vein  disappears,  the  remainder  of 
it,  as  far  forward  as  the  renal  vein,  persisting  as  the  upper 
part  of  the  left  spermatic  (ovarian)  vein,  which  thus  comes 
to  open  into  the  renal  vein  instead  of  into  the  vena  cava  as 


n 


3|         f 


286 


THE    DEVEI.Ol'MKNT   OK    THE    HUMAN    I'.ODY. 


does  tlif  corrc'spomliiig  vein  of  the  riK'Iit  side  of  the  body 
(fig.  150,  C,  sp).  The  renal  veins  originally  open  into  the 
e-'rdinals  at  the  point  where  these  aie  joined  by  tIh'  large 
cross-connetftion,  and  when  the  lower  part  of  tlie  left 
cardinal  disappears,  this  cross-connection  forms  the  prox- 
imal part  of  the  left  renal  vein,  which  coiise(iuently 
receives  the  left  suprarenal  'Fig.  150,  C). 

The  observations  upon  which  the  above  description  is 
based  have  been  made  most  thoroughly  upon  the  rabbit, 
but  it  seems  probable  from  the  partial  observations  that 
havv*  been  made  that  the  same  changes  occur  also  in  the 
human  embryo.  It  will  be  noted  from  what  has  been 
said  that  the  inferior  vena  cava  is  a  composite  vessel,  con- 
sisting of  at  least  four  elements:  (i)  the  proximal  part 
of  the  ductus  venosus;  (2)  the  anterior  part  of  the  right 
subcardinal;  (3)  the  cross-connection  between  the  right 
cardinal  and  subcardinal;  and  (4)  the  posterior  part  of 
the  right  cardinal. 

The  fate  of  the  anterior  portions  of  the  cardinal  veins 
has  yet  to  be  considered.  When  the  large  cross-connection 
with  the  subcardinals  has  been  established,  the  portions  of 
the  cardinals  iimnediately  anterior  undergo  degeneration, 
so  that  their  anterior  portions  become  quite  disconnected 
from  the  posterior  (Fig.  130,  B).  They  continue  to  re- 
ceive the  intercostal  veins,  and  the  right  onj,  retaining  its 
connection  with  the  ductus  Cuvieri,  becomes  the  vena 
azygos  (Figs.  150,  B  and  C,  va),  while  that  on  the  left  side, 
after  developing  a  cross-connection  with  its  fellow,  degen- 
erates at  its  anterior  end,  and,  so  becoming  separated 
from  the  ductus  Cuvieri,  is  transformed  into  the  vena 
hemiazygos  of  adult  anatomy  (Fig.  150,  B  and  C,  vha). 

The  ascendiiijj  lumbar  veins,  litqueiilly  described  as  the 
commencements  of  the  azygos  veins,  are  in  reality  secondary 
formations  developed  by  the  anastomoses  of  an{eriorly  and 
posteriorly  directed  branches  of  the  lumbar  veins. 


TIIK    VKINS. 


.•87 


rii(  Ihvdopmnit  oj  tin  Viius  oj  the  Limbs.  TIk'  (Uyil 
opmcnt  of  tlu'  limh  viiiis  of  tlu-  huniaii  «.'tnl)ryo  ri(|uiris 
furtlKT  invtstiKalion,  but  from  a  comparison  of  what  is 
known  witli  what  has  hctn  observed  in  rabl)it.cmbry()s  it 
may  be  i)resume(l  that  the  changes  which  take  place  are 
somewhat  as  follows.  The  blood  brouKht  to  the  limbs  by 
the  arteries  is  collected  into  a  marginal  veiti  which  sur- 
rounds the  free  edu^s  of  the  distal  portions  of  the  limb 
(Fiu.  151,  A)  and  i)asses  i)roximally  in  two  stems,  one 
situated  on  ..  •    ilnar  (fibular)  and  tlu  other  on  the  radial 


I'lc.  151.     Thk  1)i:vi;i.oi'mi:nt  oi'  riii;  \rm  \  ;:ins  in  tiik  R.vbhit. 
vl,    Vena  l)asilica;   vc,  vena  ceplialica.      U    is   to    l.c    u..'f<l    that    in    tlie 
'     ral.l-it    the   l)asilic  vein   at  .me  staj;e   (<  )  is   imu'li   rediiee.l   in   si/e, 
l)Ul  is  later  re-established.     {I  loch  stiller.) 

(tibial)  side.  In  the  anterior  extremity  the  radial  vein 
becomes  of  less  and  less  importance  (Fi^.  151.  I^^-  ^"'^  ^^ 
the  digits  develop  the  marginal  vein  becomes  broken  up 
into  segments  and  disappears  (Fig.  151,  C),  while  the 
ulnar  vein  persists,  forming  the  basilic  vein  (vb)  of  adult 
anatomy,  of  which  the  axillary  and  subclavian  veins  are 
the  proximal  continuation.  All  other  veins  of  the  arm 
are  secondary  or  tertiary  developments,  the  cephalic  (vc) 
and  other  superficial  veins  first  develo'  tng  and  'ater  the 
deep  veins   [vouc  comites).     At  fir  .  the  cephalic  vein, 


288 


riiK  i>KVKi.oi'MK\r  or   rmc  human   uodv. 


passing;  over  tlit-  clavicle,  ciiiptits  into  tlu  cxUriial  juj,'u- 
lar,  but  later  it  forms  a  conticction  with  the  axillarv  lulow 
the  clavicle,  the  port  above  this  connection  persisting; 
as  a  small  vein  known  as  the  juijulo-cephalic. 

In  the  lower  limb  the  chan^t's  are  somewhat  similar,  the 
tibial  and  marj^Mnal  veins  disappearinj;.  while  the  fibular 
persists  as  the  short  saphenous  and  sciatic  veins,  which 
arc  at  first  continuous.  The  anterior  tibial  and  lonj; 
saphenous  are  of  secondary  development,  while,  as  in  the 
.irm,  the  deep  veins  are  the  latest  to  form.  On  the  '  stab- 
lishmentof  tlicse  last  the  short  saphenous  makes  connec- 
tion with  the  popliteal,  while  the  sciatic,  like  the  corre- 
spondinjr  artery,  undergoes  a  marked  reduction. 

The  Pitlmoiuiry  Veins.~"i\\c  development  of  the  pul- 
monary veins  has  already  been  descril>ed  in  connection 
with  the  development  of  the  heart  (see  p.  -54). 

The  Fetal  Circulation. — During  fc*  life  while  the 
placenta  is  the  sole  organ  in  which  occur  the  changes  in 
the  blood  on  which  the  nutrition  of  the  embryo  depends, 
the  course  of  the  blood  is  necessarily  somewhat  different 
from  what  obtains  in  the  child  after  birth.  Taking  the 
placenta  as  the  starting-point,  the  blood  passes  along  the 
umbilical  vein  to  enter  the  bod^uif  Jlie  fetus-aUJiiuiinbili- 
cus,  whence  it  passes  forward  in  the  free  edge  of  the  ante- 
rior mesentery  (see  p.  340)  until  it  reaches  the  liver.  Here, 
owing  to  the  anastomoses  between  the  umbilical  and 
omphalo-mesenteric  veins,  a  portion  of  the  blood  traverses 
the  substance  of  the  liver  to  open  by  the  hepatic  veins  into 
the  inferior  vena  cava,  while  the  remainder  passes  on 
through  the  ductus  venosus  to  the  cava,  the  united  streams 
opening  into  the  right  auricle.  This  blood,  whose  purity 
is  only  slightly  reduced  by  ..Kture  with  the  blood  re- 
turning from  the  inferior  vena  cava,  h:  prevented  from 
passing  into  the  right  ventricle  by  the  Eustachian  valve, 


li 


''^M^ 


-w 


:J-,.g.JIW 


IIIK    l-KTAI.   CIKCUI.ATION.  2^ 

Which  directs  it  n.  the  foramen  ovale,  and  through  this  it 
passes  int..  the  left  auricle,  tl,.  .ce  tc.  the  left  ventr.cle.and 
so  out  bv  the  systemic  aorta. 


ml 


n 


Inc.  1.S2.  -Thk  Fetal  Circulation-. 
„„     \<,rta-    .i /.iMmlnionurv   urtery;    >iu,  unvhilical    artery;    </a,    ductus 
'    arteri.'.sus;  iv    .hut us  ven..s,.s:  ,«/,  intestine ;  vn  and  ^'".  '"f';"';; 
and    superior  vena   cava;    rh,  hepatic  vein;    r  ,  vena  porta;,    t./'«. 
pulmonary  vein;r»,  uinl)ilical  vein— (/•  nw  hollmann.) 

The  blood  which  has  been  sent  to  the  head,  neck,  and 
upper  extremities  is  returned  by  the  superior  vena  cava 
also  into  the  right  auricle,  but  this  descending'  stream 


290 


THE    DEVELOPMENT   OF    THE    HUMAN    liODV. 


,1 


ii 


ii 


opens  into  the  auricle  to  the  right  of  the  annulus  of  Vieus- 
sens  (see  Fig.  131)  and  passes  directly  to  the  right  ventri- 
cle without  mingling  to  any  great  extent  with  the  blood 
returning  by  way  of  the  inferior  cava.     From  the  right 
ventricle  this  blood  passes  out  by  the  pulmonary  artery, 
but  the  lungs  at  this  period  are  collapsed  and  in  no  condi- 
tion to  receive  any  great  amount  of  blood,  and  so  the 
stream  passes  by  way  of  the  ductus  arteriosus  into  the 
systemic  aorta,  meeting  there  the  placental  blood  just 
below  the  point  where  the  left  subclavian  artery  is  given 
off.     From  this  point  onward  the  aorta  contains  only 
mixed  blood,  and  this  is  distributed  to  the  walls  of  the 
thorax  and  abdomen  and  to  the  lungs  and  abdominal 
viscera,  the  greater  part  of  it,  however,  passing  off  in  the 
hypogastric  arteries  and  so  out  again  to  the  placenta. 

It  will  be  perceived  that  although  no  portion  of  the 
body  receives  absolutely  pure  placental  blood,  yet  the 
quality  of  that  which  is  supplied  to  the  liver,  heart,  head, 
neck,  and  upper  limbs  is  much  better  than  that  distrib- 
uted by  the  branches  arising  from  the  aorta  below  the 
union  of  the  ductus  arteriosus.  Hence  it  is  that  the  an- 
terior portions  of  the  fetus  are  nuich  better  developed 
than  the  posterior. 

At  birth  the  lungs  at  once  assume  their  functions,  and 
on  the  cutting  of  the  umbilical  cord  all  communication 
with  the  placenta  ceases.  vShortly  after  birth  the  foramen 
ovale  closes  more  or  less  perfectly,  and  the  ductus  arte- 
riosus diminishes  in  size  as  the  pulmonarv arteries  increase, 
and  becomes  eventually  converted  into  a  fibrous  coid.' 
The  hypogastric  arteries  diminish  greatly,  and  after  they 
have  passed  the  bladder  are  also  reduced  to  fibrous  cords, 
a  fate  likewise  shared  by  the  umbilical  vein,  which  be- 
comes converted  into  the  rouml  liqamcit  of  the  liver 

J 


THE    LYMPHATICS. 


291 


The  Development  of  the  Lymphatic  System.     It  has 

already  been  seen  (p.  243)  that  the  lymphocytes  first  make 
their  appearance  in  the  tissues  surroundmg  the  early 
blood-vessels,  but  opinions  differ  as  to  their  exact  origin. 
According  to  some  observers,  they  are  formed  by  modifi- 
cation of  mesenchyme  cells,  while  others  believe  that  they 
have  evidence  that  the  lymphocytes  of  the  intestinal  and 
tonsillar  lymph-nodes  are  derived  from  the  intestinal  and 
tonsillar  epithelium,  and  quite  recently  it  has  been  main- 
tained that  the  epithelial  cells  which  form  the  thymus 
bodv  in  fishes  are  directly  transformed  into  lymphocytes. 
Which  view  will  prove  correct  must  be  left  for  future  ob- 
servations to  decide. 

The  development  of  the  lymphatic  vessels  has  recently 
been  carefully  studied  in  pig  embryos  and  the  results  ob- 
tained have  bee  u  partially  confirmed  in  human  embryos 
(Sabin).     The  vessels  are  first  distim;uisliable  in  pig  em- 
bryos of  14.5  mm.  as  two  small  sacs  or  lymphJieaxis ,  which 
arise,  one  on  each  side,  from  near  the  junction  of  the  sub- 
clavian and  jugular  veins,  the  opening  of  the  sac  into  the 
veins  being  guarded  by  a  valve  due  to  the  oblique  direc- 
tion taken  by  the  outgrowth.     From  each  lympli  heart 
branches,  which  anastomose  and  radiate  in  all  directions, 
grow  outward  toward  the  skin  which  they  reach  in  em- 
bryos of  about  18  mm.,  and  in  later  stages  continue  to 
extend  in  a  radiating  manner  until  they  form  a  subcu- 
taneous network  over  the  anterior  half  of  the  body.     In 
the  mean  time  the  lymph  hearts  have  separated  from  their 
points  of  origin  (Fig.  153.  A,  ALH),  with  which,  however, 
they  remain  connected  by  a  duct,  and  from  this  a  branch 
grows  Imckward,  following  the  line  of  the  vagus  nerve  (Fig 
1 53,  A,  TD).    The  branch  on  the  left  side  soon  meets  with 
the  aorta  and,  using  this  as  a  guide,  grows  more  rapidly 
than  its  fellow  on  the  right  and  becomes  the  thoracic  durf, 


292 


THE    DEVELOPMENT    OF    THE    HUMAN    BODY. 


or,  rather,  since  it  divides  just  before  it  reaches  the  aorta 
and  sends  a  branch  backward  on  either  side  of  that  vessel, 
it  gives  rise  to  two  thoracic  ducts  (Fig.  153,  B). 


-4  B 

Fig.   153— Diagrams  shovving  tub  Akran.jement  ok  the  Lymphatic 

\  ESSULS  I\  Pk;   h.MliRYOS  OF  (.4)   20  MM.  AND  {H)   40  MM 

.irr  Jugular  vein;  ADR,  suprarenal  body;   ALII,  ante-ior  lymph  heart  • 
.l.>  aorta;  .  Irm./;,  deep  Iviiiphatics  to  the  arm;  /J,  diaphragm ;  Du, 
[•[  n^v'%     ''""^'""'";   ^l  '  .ft'"""-al  vein;  //.  branches  to  heart;  A" 
kidney,   /.,,;/>,  deep  lymphatics  to  leg;  Lu,  branches  to  lung-    \II> 
t)ranches   to    mesenteric   plexus;    Hi,    branch   t<.  oesophagus;    PCv' 

chvh,  kll  ,  right  lymphatic  duct;  .SVl',  subclavian  vein;  SV,  sciatic 
vein;    .S/    branches  to  stomach;    77>,  thoracic  duct:    II'«,  W(.lffian 


\ 


W 


THE    LYMPHATICS. 


293 


In  embryos  of  20  mm.  a  second  pair  of  lymph  hearts 
develops  at  the  junction  of  the  sciatic  veins  with  the  cardi- 
nals (Fi?.  1 53.  A,  PLH),  and  from  these  branches  grow  to- 
ward the  surface  and  radiate  subcutaneously,  similarly   o 
those  from  the  anterior  hearts,  with  which  they  eventual  y 
unite      The  thoracic  ducts,  continuing  to  elongate  back- 
ward dilate  opposite  the  kidneys  to  form  two  rcceptacula 
chyli  (Fig  1 53,  B,  ieC)  and  still  more  posteriorly  unite  with 
the  posterior  lymph  hearts,  which  then  separate  com- 
pletely from  the  veins  from  which  they  originated. 


Vxc,     154.-DEVEI.OPING    Lymphatic  Gland   ,.kom   thk   Axilla  ok  an 
Kmbryo  of  Eleven  Weeks.— (C/zicri/;.) 

In  later  stages  branches,  arising  as  outgrowths  from  the 
thoracic  ducts,  gradually  invade  the  mesentery  and  the 
various  organs,  following  in  general  the  course  of  the  arte- 
ries, as  do  also  the  branches  which  pass  to  the  limbs  to 
form  their  deep  lymphatics;  the  superficial  branches,  on 
the  contrary,  follow  essentially  the  course  of  the  veins. 
The  lymph  hearts  gradually  elongate  as  development 
proceeds  and  eventually  become  undistinguishable  from 
the  vessels,  and  at  various  points  in  the  system  minute 
plexuses  arise,  around  which  the  adjacent  mesenchyme 


m 


294  THE    DEVELOPMENT   OF   THE    HUMAN    BOny! 

condenses  to  form  a  capsule,  the  whole  constitutinij  a 
lymph-node  (V\g.  154).  ** 

Up  to  this  stage  of  the  dc  velopment  no  valves  are  pres- 
ent in  the  vessels,  and  the  development  of  these  has  yet 
to  be  studied,  as  has  also  the  final  transformation  of  the 
condition  described  into  that  found  in  the  adult     It  seems 
probable  that  in  human  embryos  the  two  thoracic  ducts 
together  with  the  receptacula  chyli,  gradually  approach' 
one   another  and   finally  fuse   throughout   their  entire 
length  to  form  the  single  receptaculum  and  thoracic  duct 
of  the  adult.     The  not  infrequent  occurrence  of  a  partial 
doubling  o    the  thoracic  duct  receives  a  simple  explana- 
tion if  this  be  Die  case. 


LITERATURE. 


C. 

J. 

J. 

A. 

W 
I 


V. 
F. 


.  ^■;;'  "•^-';'"^>;an,l  C.J,x-n:  '•  Kecherd.cssur  la  fur„.ati,.„  .Ics  annexes 
fa  talcs  Chez  les  n.anuniR.res."  Anlnvrs  d,  lUoh,^.,  v    1884 

H.  0.n.;vm :  "  Znr  .\nat..n.ie  einiser  I.ymphdrusen  in,  crwachsonen  nnd 

fotalc../nsta„cle/^l../,n./,V.J„./.,o,,//V^■,s,V,/....l„«/.l;^"88 
I).ss.:      I,,e  Kntstehung  des  Blutcs  und  der  ers;en  Gef  i^]„^  ^„. 

ncrn,"  Arclnv  jur  m.krosk.  .\„at.,  xvi,  1879 
C.    K    IjTKKNon:  •IVemiers  stades  de  la  circulation  sanguine  d-u,s 

I  unf  et  1  e.nl.ry.m  luunain,"  .1  ,u,t.  A  ,nn^rr  xv    1  H»<) 

I   u,sT...TTUR       Leber  die  ursprungliche  Hauptsehlagader  der  l.interen 
Ghedmasse  des  .Menschen  ,.nd  der  Si.ugethiere,  nehst  Hen  erkun; 
ul.er  d.e  I-.n.w.cklun,  der  lin.iaste  der  A.rta  abdonMnalis.'  T/.^m^^^^ 
Jdhrhuch,  XVI,  1890  '     ■^""^P"'"- 

"d'^'TS^B^Iir  "'^  '^"twicklungder  A.  vertehralis  hein,  Kunin- 

Hochstettek:  ''Ueberdie  Kntwicklung  der  Kxtremi.atsvenen  bei  den 
Aninioten,"  Morbid.  Jahrbuch,  xvii    1891 

.en,s  der  An,ni„ten,"  Morphol.  JakrbnchX^^X^^t      ''^  '  """^■"^- 


LITKKATL'KK. 


^95 


V. 


c. 

H 
V. 
(). 
(». 

'/A 


II.  IIowKUi,:  ■Tlic  Life-history  of  llicl'onm-d  rnomeiits  of  tlu  Ml I, 

lispccially  tlic  Red  Hlood-corpusck-s,"  Jo..rn.  oj  Moiphol.,  iv,  WM. 
n    HowKi.!,:    ■Ol.scrvations  on  tl        ccuruncc,  Structure,       il  I'unc- 

tion  of  the  Giant-cells  of  the  M.  ..ow,"  Jouni.  oj  Morpiwi      v,  IH'm. 
P.  Malu;  ■•  Development  of  the  Interni-l  Maniniary  and  Deep  ICpisastnc 

Arteries  in  Man,"  Johns  Hopkins  IlospiUil  lUdhtin,  I8<)8. 
NrsBAi  M  and  T    Fkymak:  "Zur  Kiitwickehu.-iSK'eseliiehte  der  lyni- 

phoidcn  I'leniente  der  Thymus  I.ei  ..en  Knoehenfischen,"  .\iiat.  .!»/- 

r,,,i;rr,  XIX,  IWl. 
RivTTUkKR:  "Sur  la  part  que  preiul  I'epilluliuni  a  la  formation  de  la 
bourse  de  Fahiicius,  des  aniy',dales  et  des  plaques  de  I'eyer,"  Joiiru. 
dc  I'Amit.  ct  dc  la  Physiol.,  XXIX,  1893. 
Rosi- :  "  Zur  lint  wicklungsgcschichte  des  Sauge!.hierher;xns,"  Morphol. 

liiUrhuch,  XV.  1889. 
SA1.ZKK:  "Uelar  die  l{ntwicklunK  der  Kopfvenen  des  Meersdiwein- 
chens,"  Morphol.  Jnhrhitch,  xx,  189,? 
Stoiik:  "Ueherdie  luitwickhnig  der  D.undyinphknoteheii  undiiherdie 

Riickbildung  von  Darmdriisen,"  Anhiv  jur  mikro.^k.  Awit.,  U,  1898. 
VAN  I)i;r  .Stkicht:  "Xouvelles  n-clierches  snr  la  genese  des   globules 

rouges  et  des  globules  blancs  du  sang,"  .\nhhrs  dc  lUolo^.,  xii,  1892. 
VAN  i>i:k  Stkicht:  "De  la  premiere  on„a.ie  tin  sang  et  des  oapillaires 
sanguins  dans  I'aire  vasculaire  du   I.apin,  '  Comptcs  k'ludiis  dc  la  Soc. 
dc  Biolof^.  Paris,  Ser,  10,  ll,  189.S. 
mmKkmann:  "Ueber   die    Kiemenarterienbogen  des     Minschen,"    \'cr- 
handl.  des  Xtcn  urtrnuit.  mcilic.  (^'oyv^rcsscs^  ll,  1891. 


I) 


■f 


CIIAI'TKR    X. 

THE    DEVELOPMENT    OF    THE    DIGESTIVE 
TRACT  AND   GLANDS. 

The  greatest  portion  of  the  digestive  tract  is  formed  by 
the  constriction  off  of  the  dorsal  portion  of  the  yolk-sac, 
as  shown  in  Fig.  39.  the  result  being  the  formation  of  a 
cylinder,  closed  at  either  end,  and  composed  of  a  layer  of 
splanchnic  mesoderm  lined  on  its  inner  surface  by  endo- 
derm  This  cylinder  is  termed  the  archenteron  and  has 
connected  with  it  the  volk-stalk  and  Ihe  allantois,  the 
latter  communicating  witl  its  somewhat  dilated  termmal 
portion,  which  also  receives  the  ducts  of  the  primitive 
kidneys  and  is  known  as  the  cloaca  (Fig.  156). 

At  a  very  earlv  stage  of  development  the  anterior  end 
of  the  embryo  begins  to  project  slightly  in  front  of  the 
yolk-sac,  so  that  a  shallow  depression  is  formed  between 
the  two  structures.     As  the  constriction  of  the  embryo 
from  the  sac  proceeds,  the  anterior  portion  of  the  brain 
l^ecomes  bent  ventrally  and  the  heart  makes  its  appear- 
ance immcdiatelv  in  front  of  the  anterior  surface  of  the 
yolk-sac,  and  so  the  depression  mentioned  above  becomes 
deepened  (Fig.  155)  to  form  the  oral  sinus.     The  floor  of 
this,  Uned  by  ectoderm,  is  immediately  opposite  the  ante- 
rior end  of  tlu'  archenteron,  and,  since  mesoderm  does  not 
develop  in       s  region,  the  ectoderm  of  the  sinus  and  the 
cndoderm  of  the  archenteron    are    directly  in  contact, 
forming  a  thin  pharyngeal  membrane  separating  the  two 
cavities  (Fig.   155,  /"")•     ^^  embryos  of  2.15   mm.  this 
membrane  is  still  existent,  but  soon  after  it  becomes  per- 

296  * 


THE    DIGESTIVE   TRACT. 


297 


forated  and  finally  disappears,  so  that  the  archenteron 
and  oral  sinus  becone  continuous. 

Toward  its  posterior  end  the  archenteron  conies  into 
somewhat  similar  relations  with  the  ectoderm,  though  a 
marked  difference  is  noticeable  in  that  the  area  over  which 
the  cloacal  endoderm  is  in  contact  with  the  ectoderm  to 
form  the  cloacal  membrane  (Fig.  156,  cm)  lies  :i  little  in 
front  of  the  actual  end 
of  the  archenteric  cylin- 
der, the  portion  of  the 
latter  which  lies  poste- 
rior to  the  membrane 
forming  what  has  been 
termed  the  post-anal  (jut 
(p.an).  This  diminishes 
in  size  during  develop- 
ment and  early  disap- 
pears altogether,  and  the 
pouch-like  fold  seen  in 
Fig.  156  between  the  in- 
testinal portion  of  the 
archenteron  and  the  al- 
lantoic stalk  (al)  deepen- 
ing until  its  floor  comes 
into  contact  with  the 
cloacal    membrane,    the 

cloaca  becomes  divided  into  a  ventral  portion,  with 
which  the  allantois  and  the  primitive  excretory  ducts  (w) 
are  connected,  and  a  dorsal  portion  which  becomes  th.e 
lower  end  of  the  rectum.  This  latter  abuts  upon  the 
dorsal  portion  of  the  cloacal  membrane,  and  this  event- 
ually ruptures,  so  that  the  posterior  communication  of 
the  archenteron  with  the  exterior  becomes  established. 
This  rupture,  however,  does  not  occur  until  a  compara- 
25 


vlo.  i.s5.  '...(.onstkuction  ok  the 
Antekiuk  Portion  of  an  I{mbkyo  ok 

2.15    MM. 

(i/<,  A.irtic  bulb;  /;,  licart ;  o,  atiditory 
capsule;  ofy,  optic  evaginalion;  (^m, 
pharyngeal  iiiciiibrane. — {His.) 


MM 


i 


298 


THE    DEVELOPMENT  OF    THE    HUMAN   BODY. 


tively  late  period  of  development,  until  after  the  embryo 
has  reached  the  fetal  stage;  nor  does  the  position  of  the 
membrane  correspond  with  the  adult  anus,  since  later 
there  is  a  considerable  development  of  mesoderm  around 
the  lower  end  of  the  rectum,  which  bulges  out,  as  it  were, 
the  regions  immediately  surrounding  the  membrane,  pro- 


/tc 


FlC.    156.  —  RECUNSTKt  CTION   Ol"    1111;    HiM)   K.NI>  OK   AN    lilMBRYO  6.5    MM. 

LONd. 

a/,  Allantois;  h,  hdly-stalk ;  W,  cloaca;  cm,  cloacal  membrane;  i,  intes- 
tine; w,  spinal  curd;  nc,  notuchord;  p.an,  post-anal  gut;  nr,  out- 
growth to  form  ureter  and  metanf[)hros;  w.  Wolffian  duct. — {Keihel.) 

ducing  a  short  ectodermal  addition  to  the  rectum,  the 
end  of  which  is  the  definitive  anus. 

It  will  be  noticed  that  the  digestive  tract  thus  formed 
consists  of  three  distinct  portions,  an  anterior,  short,  ecto- 
dermal portion,  an  endodermal  portion  representing  the 
original  archenteron,  and  a  posterior  short  portion  which 
is  also  ectodermal.     The  differentiation  of  the  tract  into 


ariliua 


TllK    MOUTH-CAVITY. 


299 


its  various  regions  and  the  formation  of  the  various  organs 
found  in  relation  with  these  may  now  be  considered. 

The  Development  of  the  Mouth  Rej?ion.-The  decpenmg 
of  the  oral  sinus  by  the  development  of  the  first  branchial 
arch  and  its  separation  into  the  oral  and  nasal  cavities 
by  the  development  of  the  palate  have  already  been  de- 
scribed (p.  103),  but,  for  the  sake  of  continuity  in  descrip- 
tion, the  latter  process  may  be  briefly  recalled.  At  hrst 
the  nasal  pits  communicate  with  the  oral  sinus  by  grooves 
lying  one  on  each  side  of  the  fronto-nasal  process,  but  by 


Fig   157  ^Vmw  of  thk  Roof  ok  the  Oral  Fossa  of  ICmbrvo  shovvino 

THE   LUMIKOOVE   AN.)   THE    FORMATION   OF   THE    PALATE.-(//ts.) 

the  union  of  the  latter  with  the  maxillary  processes  this 
communication  is  partly  interrupted,  though  the  pits  still 
retain  connection  with  the  oral  sinus  behind  the  maxillary 
process.  At  about  the  fifth  week  a  downgrowth  of  epi- 
thelium into  the  substance  of  both  the  maxillary  and 
frontonasal  processes  above  and  the  mandibular  process 
below,  takes  place  and  the  surface  o.  the  downgrowth 
becomes  marked  by  a  deepening  groove  (Fig.  157).  which 
separates  an  anterior  fold,  the  lip,  from  the  jaw  proper 
(Fig.    158).     From  the  premaxillary  and  maxillo-pala- 


Itil 

(I'  ^ 

[  ^ 

i.  V-  1 

■' 1 


300 


THE    DEVELOPMENT   OK    THE    HUMAN    BODY. 


tine  portions  of  the  upper  jaw,  shelf-like  ridges  then  begin 
to  grow  backward  and  inward,  and  at  about  the  beginning 
of  the  third  month  these  meet  in  the  median  line  to  form 
the  palate,  completing  the  separation  of  the  definitive 
mouth  from  the  nasal  cavity.  At  the  point  of  meeting 
of  the  premaxillary  and  maxillary  shelves  a  small  com- 
munication between  the  two  cavities  persists  for  a  time, 
frequently  until  after  birth ;  it  allows  passage  of  the  ante- 
rior palatine  vessels  and  nerves,  and  places  the  organ  of 
Jacobson  (p.  457)  in  communication  with  the  mouth. 
Later  the  opening  becomes  closed  over  by  mucous  mem- 
brane, but  it  may  be  recognized  in  the  dried  skull  as  the 
fora   len  incisivum  (anterior  palatine  canal). 

Before  the  formation  of  the  palate  begins,  a  pouch  is 
formed  in  the  median  line  of  the  roof  of  the  oral  sinus,  just 
in  front  of  the  pharyngeal  membrane,  by  an  upgrowth  of 
the  epithelium.  This  pouch,  known  as  Rathke's  pouch, 
comes  in  contact  above  with  a  downgrowth  from  the  floor 
of  the  brain  and  forms  with  it  the  pituitary  body  (see  p. 
418). 

The  Development  of  the  Teeth. — When  the  epithelial 
downgrowth  which  gives  rise  to  the  lip  groove  is  formed,  a 
horizontal  outgrowth  develops  from  it  which  extends 
backward  into  the  substance  of  the  jaw,  forming  what  is 
termed  the  denial  shelf  (Fig.  158,  A).  This  at  first  is  situ- 
ated on  the  anterior  surface  of  the  jaw,  but  with  the  con- 
tinued development  of  the  lip  fold  it  is  gradually  shifted 
until  it  comes  to  lie  upon  the  free  surface  (Fig.  158,  B), 
where  its  superficial  edge  is  marked  by  a  distinct  groove, 
the  dental  groove  (Fig.  157).  At  first  the  dental  shelf  of 
each  jaw  is  a  continuous  plate  of  cells,  uniform  in  thick- 
ness throughout  its  entire  width,  but  later  ten  thickenings 
develop  upon  its  deep  edge,  and  beneath  each  of  these  the 
mesoderm  condenses  to  form  a  dental  papilla,  over  the 


'^"— '" 


TIIK    TF.ETII. 


30  > 


surface  of  which  the  tliickcning  moulds  itself  to  form  a 
cap  termed  the  enamel  organ  (Fi^'-  158.  B).  These  ten 
papilla  in  each  jaw.  with  their  enamel  caps,  represent  the 
teeth  of  the  first  dentition. 

The  papilla?  do  not,  however,  project  into  the  very  edge 
of  the  dental  shelf,  but  obliquely  into  what,  in  the  lower 


.■:  -VV-..-A.,;-;  ?.-.:'.ViV--:'v.*>  ■ 


Hy:^-/^ 


Fi<;    158-  TransveksE  Sections  thro.-gh  thf  t,o,. 
THE  Formation  ok  the  Dental  Shelk  in  Kml 

AND  {B)  40  MM.— (A'ti.?t'.) 


'  '  \V    SIIOWINC 
(.1)    17    MM. 


jaw,  was  originally  its  under  surface  (Fig.  1 58,  B),  so  that 
the  edge  of  the  shelf  is  free  to  grow  still  deeper  mto  the 
surface  of  the  jaw.  This  it  does,  and  upon  the  extension 
so  formed  there  is  developed  in  each  jaw  a  second  set  of 
thickenings,  beneath  each  of  which  a  dental  papilla  agam 
appears.  These  looth-gcrms  represent  the  incisors,  jca- 
nines,  and  premolars  of  the  permanent  dentition. 


The 


!» 


302 


THE    UEVELOI'MENT   OK    THE    HUMAN    HOnV. 


lateral  t'd^cs  of  the  dental  shelf  being  continued  outward 
toward  the  articulations  of  the  jaws  as  prolongations 
which  are  not  connected  with  the  surface  epithelium,  op- 
portunity is  afforded  for  the  development  of  three  addi- 
tional thickenings  on  each  .  .dc  in  each  jaw,  and,  papilhe 
developing  beneath  these,  twelve  additional  tooth-germs 
are  formed.  These  represent  the  permanent  molars; 
their  formation  is  much  later  than  that  of  the  other  teeth, 
the  germ  of  the  -iccond  molar  not  appearing  until  about 
the  sixth  wexk  after  birth,  while  that  of  the  third  is  de- 
layed until  about  the  fifth  year. 

As  the  tooth  germs  increase  in  size,  they  approach 
nearer  and  nearer  to  the  surface  of  the  jaw,  and  at  the 
same  time  the  enamel  organs  separate  from  the  dental 
shelf  until  their  connection  with  it  is  a  mere  neck  of  epithe- 
lial cells.  In  the  mean  time  the  dental  shelf  itself  has  been 
undergoing  degeneration  and  is  reduced  to  a  reticulum 
wliicli  eventually  completely  disappears,  though  frag- 
ments of  it  may  occasionally  persist  and  give  rise  to  vari- 
ous malformations.  With  the  disappearance  of  the  last 
remains  of  the  shelf,  the  various  tooth-germs  naturally 
lose  all  connection  with  one  another. 

It  will  be  seen,  from  what  has  been  said,  that  each  tooth- 
germ  consists  of  two  portions,  one  of  which,  the  enamel 
organ,  is  derived  from  the  ect'  :lerm,  w^hile  the  other,  the 
dental  papilla,  is  mesenchymatous.  Each  of  these  gives 
rise  to  a  definite  portion  of  the  fully  formed  tooth,  the 
enamel  organ, as  its  name  indicates,  producing  the  enamel, 
while  from  the  dental  papilla  the  dentine  and  pulp  arc 
formed. 

The  cells  of  the  enamel  organ  which  are  in  contact  with 
the  surface  of  the  papilla,  at  an  earlv  stage  assume  a 
cylindrical  form  and  become  arranged  in  a  definite  layer, 
the  enamel  niemhranc  (Fig.  159,  SF2i),  while  the  remaining 


1)1 


rilK   TEETH. 


303 


cells  iSKa)  apparently  degenerate  eventually,  thougli 
they  persist  for  a  time  to  form  what  has  been  termed  the 
vmimel  pulp.     The  formation  of  the  enamel  seems  to  be 


-     v. 


PiP,   1  so  —Section  THKOidH  the  I'irst  Mouak  Tooth  or  a  Rat,  Twei.vk 

Days  Old. 

.1/,.  Periosteum:  E,  dentine;  £/.,  epidermis ;  O./.  od.mtoblasts;  S,  enamel : 
SEa  and  SEi,  outer  and  inner  layers  of  the  enamel  organ  ;  .Sh,  portion 
of  the  enamel  organ  which  does  not  produce  enamel-  (row  bnititi.) 

due  to  the  direct  transformation  of  the  enamel  cells,  the 
process  beginning  at  the  basal  portion  of  each  cell,  and 
as  a  result,  the  enamel  consists  of  a  series  of  prisms,  each 


'>r   1 


304 


THE    DEVELOPMENT   OF    THE    HUMAN    BODY. 


of  whicli  represents  one  of  the  cells  of  the  enamel  mem- 
brane. The  transformation  proceeds  until  the  cells  have 
become  completely  converted  into  enamel  prism.s,  except 
at  their  very  tips,  which  form  a  thin  membrane,  the 
enamel  cuticle,  which  is  shed  soon  after  the  eruption  of  the 
teeth. 

The  dental  papilla^  are  at  first  composed  of  a  closely 
packed  mass  of  mesenchyme  cells,  which  later  become 
differentiated  into  connective  tissue  into  which  blood- 
vessels and  nerves  penetrate.     The  superficial  cells  form 
a  more  or  less  defmite  layer  (Fig.  159,  od),  and  are  termed 
odontoblasts,  having  the  function  of  manufacturing  the 
dentine.     This  they  accomplish  in  the  same  manner  as 
that  in  which  the  periosteal  osteoblasts  produce  bone,  de- 
positing the  dentine  between  their  surfaces  and  the  adja- 
cent surface  of  the  enamel.     The  outer  surface  of  each 
odontoblast  is  drawn  out  into  a  number  of  exceedingly 
fine  processes  whicli  extend  into  the  dentine  to  occupy 
the  minute  dentinal  tubules,  just   as   processes  of  the 
osteoblasts  occupy  the  canaliculi  of  bone. 

At  an  early  stage  the  enamel  membrane  forms  an  al- 
most complete  investment  for  the  dental  papilla  (Fig. 
159),  but,  as  the  ossification  of  the  tooth  proceeds,  it  re- 
cedes from  the  lower  part,  until  finally  it  is  confined  en- 
tirely to  the  crown.  The  dentine  forming  the  roots  of  the 
tooth  then  becomes  enclosed  in  a  layer  of  cement,  which 
is  true  bone  and  serves  to  unite  the  tooth  firmly  to  the 
walls  of  its  socket.  As  the  tooth  increases  in  size,  its  ex- 
tremity is  brougl  icarcr  to  the  surface  of  the  gum  and 
eventually  breaks  through,  the  eruption  of  the  first  teeth 
usually  taking  place  during  the  last  half  of  the  first  year 
after  birth.  The  growth  of  the  permanent  teeth  proceeds 
slowly  at  first,  but  later  it  becomes  more  rapid  and  pro- 
duces pressure  upon  the  roots  of  the  primary  teeth.     The 


I 


t 


THE   TONGUE.  3^5 

roots  of  these  then  undergo  partial  absorption  and  so  are 
loosened  in  tlieir  sockets  and  are  readily  pushed  out  by 
the  further  growth  of  the  permanent  teeth. 

The  dates  and  order  of  the  eruption  of  the  teeth  are  suhject 
to  eon 'derable  variation,  but  the  usual  sequenee  ,s  somewhat 
as  follows : 

Primary  Dentition. 

.     .  6tli  to  8th  month. 

Median  incisors . 

, .     .  7th  to  9th  month. 

Lateral  incisors .      . 

...    ,       ,  .Beginning  of  2cl  year. 

I'lrst  molars '^ 

,.     •                                                       .  .  .  1  i  years. 
Lanines,    ■*  -    , , 

Second  molars to  .^  years. 

The  teeth  of  the  lower  jaw  generally  precede  those  of  the 
upper. 

'^'^  Permanknt  Dkntition. 

,  .    "th  year. 

I<irst    molars - 

Middle   incisors «l  '  y*^"*-' 

....  .   9th  year. 

Lateral  incisors,    

i,„  .  10th  year. 

I'lrst  premolars,   

Second   premolars, '  ^  ' 

Canines,  "(^  l.^th  to   14th  years 

Second  molars,  I 

'  ,  •    .        1  .  -l^th  to  40th  years. 

Third  molars, 

Tn  n  ronsider  ible  percentage  of  individuals  the  third  molars 
(vv  ^dottXtl  )  «  -^  'hrough  the  guuts,  and  freqt.ently 

e  he  ck  o  thev  fail  to  reach  the  levelof  the  other  teeth,  and 
s  are  on  V  partly  functional.  These  and  other  pecuharities  of 
a  sJrucUtra/mtture  shown  by  these  teeth  indicate  that  they  are 
luidergoing  a  retrogressive  evolution. 

The  Development  of  the  Tongue.-Strictly  speaking, 
the  tongue  is  largely  a  development  of  the  pharyngeal 
region  of  the  digestive  tract  and  only  secondardy  grows 
forward  into  the  floor  of  the  mouth.  In  embryos  o 
about  ^  mm.  there  may  be  seen  in  the  median  hne  of 
the  floor  of  the  mouth,  between  the  ventral  ends  of  the 
first  branchial  arches,  a  small  rounded  elevation  which 
has  been  termed  the  iuberculum  impar.  In  later  stages 
26 


1:1 

m 


3o6 


THE    DEVELOPMENT   OF    THE    HUMAN    BODY. 


(Fig.  1 60,  A)  this  becomes  larger  and  reaches  its  greatest 
development  in  embryos  of  about  8  mm.,  after  which  it 
becomes  less  prominent  and  finally  unrecognizable,  but 
before  this  there  has  appeared  on  each  side  of  the  floor  of 
the  mouth  a  longitudinal  groove,  each  of  which  at  its 
anterior  end  bends  medially  toward  its  fellow.  By  these 
alveolo-linqual  grooves  an  area  is  marked  out  in  the  floor  of 
the  mouth  which  gradually  becomes  more  and  more  prom- 


FiG.  160.  -Floor  of  the  Pharynx  of  Kmbrvos  of  (.4)  7  and  (/i)  10 

MM.   SHOWI.NG  THE   DEVELOP.MEN'T  OF  THE  ToNdl'E. 

Ep,  Epiglottis;  Sp,  pra-ccrvical  sinus;  /'  and  f^,  median  and  lateral  portions 
of  the  tongue;  /  to  IV ,  branchial  arches. — {His.) 


inent  and  rounded  upon  its  oral  surface,  and  forms  the 
anterior  portion  of  the  tongue  (Fig.  160,  B,  /i).  This 
median  elevation  is  bounded  at  the  sides  and  almost  to 
the  median  line  in  front  by  the  alveolo-Hngual  grooves, 
and  posteriorly  it  is  separated  from  the  anterior  edge  of 
the  second  branchial  arch  by  a  distinct  V-shaped  groove, 
at  the  apex  of  wliich  is  a  deep  circular  depression,  the 
foramen  Cfccum  (see  p.  313). 

The  posterior  portion  of  the  loiiguc  arises  as  thickenings 


THE   TONGUE. 


307 


of  the  ventral  ends  of  the  second  branchial  arches,  and  is 
consequently  ;i  V-shaped  structure,  into  the  angle  of  which 
the  posterio.  part  of  the  anterior  portion  of  the  tongue 
fits  (Fig.  161).  The  two  portions,  anterior  and  posterior, 
eventually  fuse  together,  but  the  groove  which  originally 
separated  them  remains  more  or  less  clearly  distinguish- 
able, the  circum vallate  pa- 
pillae (see  p.  458)  develop- 
ing immediately  anterior 
to  it. 

The  tdiiguc  is  essentially  a 
nmscular  organ,  being  formed 
of  a  central  mass  of  muscular 
tissue,  enclosed  at  the  sides 
and  dorsallv  by  mucous  mem- 
l^rane  derived  from  the  floor 
of  the  mouth  and  pharvnx. 
The  muscular  tissue  consists 
partly  of  fibers  limited  to  the 
substance  of  the  tongue  and 
forming  the  m.  lingnalis,  and 
also  of  a  number  of  extrinsic 
nmscles,  the  hyoglossi,  genio-  ,     ,       ,      ,  ^«u     1     . 

hvoglossi,  styloQlossi,  palatoglossi,  and  chondroglossi.  Ihc  last 
two  muscles  are  innervated  by  the  vagus  nerve,  and  the 
remaining  extrinsic  muscles  receive  fibers  from  the  hypo- 
glossal, while  the  lingualis  is  supplied  partly  by  the  hypoglossal 
and  partly,  apparently,  by  the  facial  through  the  chorda  tym- 
pani.  That  the  facial  should  take  part  in  the  supply  is  what 
might  be  expected  from  the  mode  of  development  of  the  tongue, 
but  the  hypoglossal  has  been  seen  to  correspond  to  certam  pri- 
marily po'stcranial  metameres  (p.  192).  and  its  relation  to  struc- 
tures'* 'cing  part  in  the  formation  of  an  organ  belonging  to  the 
antei  jT  part  of  the  pharynx  seems  somewhat  anomalous.  It 
may  be  supposed  that  in  the  evolution  of  the  tongue  the  ex- 
trinsic muscles,  together  with  a  certain  amount  of  the  linguahs, 
haye  grown  into  the  tongue  thickenings  from  regions  situated 
much  further  back,  for  the  most  part  from  behind  the  last 
branchial  arch. 

Such  an  invasion  of  the  tongue  by  muscles  from  posterior 


Fig.  161.-  The  Fuoor  of  the 
Pharynx    of    an    Kmbryo    of 

ABOUT   20  MM. 

cp,  Epiglottis;  }c,  foramen  csecuin; 
<i  and  t^,  median  and  lateral  por- 
tions of  the  tongue.— (//J>.) 


3o8 


THE    DEVELOPMENT   OF    THE    HUMAN    BODY. 


segments  would  explain  the  distribution  of  its  sensory  nerves. 
The  anterior  portion,  from  its  position,  would  naturally  be  sup- 
plied by  branches  from  the  fifth  and  seventh  "erves,  while  the 
posterior   portion    might  be  expected  to  be  supplied  by  the 


•    1 


•I     f 


Fk..    162. —  DiAdRAM  OF   THE    DISTRIBUTION   OF  THE   SENSORV    NERVES  OF 

THE    TONCRE. 

The  area  supplied  by  the  fifth  (and  seventh)  nerve  is  indicated  by  the 
transverse  Hnes;  that  of  the  ninth  by  the  oblique  lines;  and  that  of  the 
tenth  by  the  small  circles. — (Zander.) 

seventh.  There  seems,  however,  to  have  been  a  d'slocation 
forward,  if  it  may  be  so  expressed,  of  the  mucous  membrane, 
the  sensory  dislrihrtifm  of  the  ninth  ntrve  extindinj;  forward 
upon  the  ptjsterior  part  of  the  anterior  portion  of  the  tongue. 


THK    SALIVARY    GLANDS. 


309 


while  a  considerable  portion  of  the  posterior  portion  is  supplied 
bv  he  tenth  nerve.  The  distribution  of  the  sensory  fibers  of 
the  facial  is  probablv  confined  entirely  to  the  anterior  p.)rtu.n 
thoueh  funhJr  nformation  is  needed  to  determine  the  exact 
Sfbutlon  ^^^^^^^^^^^^  the  motor  and  sensor>-  fibers  of  this  nerve  in 
the  tongue. 

The  Development  of  the  Salivary  QIands.-In  embryos 
of  about  8  mm.  a  slight  furrow  may  be  observed  m  the 
floor  of  the  groove  which  connects  the  lip  grooves  of  the 
upper  and  lower  jaws  at  the  angle  of  the  mouth  and  may 
be  known  as  the  cheek  groove.     In  later  stages  this  furrow 


F„;      16^  -.\N    OBLIQUE    SECTION    THROICH    THE    MoUTH    CAVITY    OF    AN 

Embryo  OF  ABOi'T  16  TO  1/  mm. 
..„   Mpokpl's  cartilage  ■  .</,  inferior  dental  nerve ;  nl,  lingual  nerve ;  /'parotid 
"^'  ^^tsT^^nlm  of  the  tongue;  ./.  sublingual  sland;  .m,  suhuiax.l- 
lary  gland ;  /,  tooth ;  A7/,  hypoglossal  nerve,     (//iv.) 

deepens  and  ually  becomes  closed  in  to  form  a  hol- 

low tubular  s  •  turc,  wliic.^  n  embryos  of  17  mm.  has 
separated  from  the  epithelium  of  the  floor  of  the  cheek 
groove  except  at  its  anterior  end  and  has  become  em- 
bedded in  the  connective  tissue  of  the  cheek.  This  tube 
is  readily  recognizable  as  the  parotid  gland  and  Stensons 
duct  and  from  the  latter  as  it  passes  across  the  masseter 
muscle  a  pouch-like  outgrowth  is  early  formed  which 
probably  represents  the  socio  parotidis. 

The  submaxillary  gland  and  Wharton  s  duct  appear  m 


3IO 


THK    DEVEI-OI'MENT   OF     THE    HL'MAN    IfODV. 


il 


■■♦        ! 


I    " 


embryos  of  about  13  nun.  as  a  longitudinal  ridge-like 
thickening  of  the  epithelium  of  the  floor  of  the  alveolo- 
lingual  groove  (see  p.  306).  This  ridge  gradually  sepa- 
rates from  behind  forward  from  the  floor  of  the  groove  and 
sinks  into  the  subjacent  connective  tissue,  retaining,  how- 
ever, its  connection  with  the  epithelium  at  its  anterior 
end,  which  indicates  the  position  of  the  opening  of  the 
duct.  In  the  vicinity  of  this  there  appear  in  embryos  of 
24.4  mm.  five  small  bud-like  downgrowths  of  the  epithe- 
lium, which  later  increase  considerably  in  number  as  well 
as  in  size,  and  constitute  a  group  of  glands  which  are  gen- 
erally spoken  of  as  the  sublingual  gland. 

As  these  representatives  of  the  various  glands  increase 
in  length,  they  become  lobcd  at  their  deeper  ends,  and  the 
lobes  later  give  rise  to  secondary  outgrowths  which  branch 
repeatedly,  the  terminal  branches  becoming  the  alveoli  of 
the  gHnds.  A  lumen  early  appears  in  the  duct  portions 
of  the  structures,  the  alveoli  remaining  solid  for  a  longer 
time,  although  they  eventually  also  become  hollow. 

It  is  t(j  be  noted  that  each  parotid  and  submaxillar}-  consists 
of  a  single  primary  outgrowth,  and  is  therefore  a  single  structure 
and  not  a  unifm  of  a  number  of  originally  separate  parts.  The 
sublingual  glands  of  adult  anatomy  are  usually  described  as 
opening  upon  the  floor  of  the  mouth  by  a  number  of  sejjarate 
ducts.  This  arises  from  the  fact  that  the  majoritv  of  tlie  glands 
which  form  in  the  vicinity  of  the  opening  ()f  Wharton's  duct 
remain  quite  small,  only  one  of  them  on  each  side  giving  rise  to 
the  sublingual  gland  proper.  The  small  glands  have  been 
termed  the  alveolo  lingual  glands,  and  each  oni  of  them  is 
equivalent  to  a  parotid  or  submaxillary  gland.  In  otlier  words, 
there  are  in  reality  not  three  pairs  of  salivary  glands,  hut  frotn 
fourteen  to  sixteen  pairs,  there  being  usually  from  eleven  to 
thirteen  alveolo-lingual  glands  on  each  side. 

The  Development  of  the  Pharynx. — The  pharynx  repre- 
sents the  most  anterior  part  of  the  archcnteron,  that  por- 
cion  in  which  the  branchial  arches  develop,  and   in   the 


i 


i 


I 


IHK    rHARVNX. 


3H 


-si- 


embryo  it  is  relatively  tnuch  longer  than  in  the  adult  e 
diminution  being  brought  about  by  the  folding  in  of  the 
posterior  arches  and  the  formation  of  the  sinus  pra^-ervi- 
calis  already  described  (p.  lo.).  Between  the  various 
branchial  arches,  grooves  occur,  representing  he  endo- 
dermal  portions  of  the  grooves  which  separate  the  arches^ 
During  development  the  first  of  these  becomes  converted 
into  the  tympanic  cavity  of  the  ear  and  the  Eustachian 

tube  (see  Chap.  XV);  the  second  disappears  in  its  upper 

part,  the  lower  persisting  as  the 

groove  of    Rosenmuller  and   the 

fossa  in  which  the  tonsil  is  situ- 
ated;   while  the  remaining  two 

disappear,  leaving  traces  of  their 

existence  in   detached   portions 

of  their  epithelium  which  form 

what   are  termed  the  branchial 

epithelial   bodies,   among   which 

are   the   thyreoid   an1   thymus 

glands. 

In  the  floor  of  the  pharynx 
behind  the  thickenings  which 
produce  the  tongue  there  is  to 
be  found  in  early  stages  a  pair 

ot  thickenings  passing  horizontally  backward  and  unit- 
ing in  front  so  that  they  resemble  an  inverted  U    Pig- 
iL   /)      These  rid-,  s,  which  form  what  is  termed  the 
fureula  (His),  are  c        erned  in  the  formation  of  parts  of 
the  larvnx  (see  p.  355)-     I"  the  part  of  the  roof  of    he 
pharviix  which  comes  to  lie  between  the  openings  of  the 
ICustachian  tubes,  a  collection  of  lymphatic  tissue  takes 
place    beneath    the    mucous     membrane,    forming     the 
phnrvnncal  tonsil  and  immediately  behind  this  there  is 
formed  in  the  median  line  an  upwardly  projecting  pouch, 


Fig.    164— The  Floor  of 
THE  Pharynx  ok  an  Km- 

BRYO  OF  2.1.S  MM. 

/,  Fureula ;  t,  median  portion 
of  tongue.— (//'>.) 


312 


THE  DKVKLOf'MENT  OF  TMK  HUMAN  BODY. 


the  pharyngeal  bursa,  first  certainly  noticeable    in    em- 
bryos 6.5  mm.  in  length. 

This  bursa  has  very  generally  been  regarded  as  the  persistent 
remains  of  Rathkc's  pouch  (p.  ^fX)),  especially  since  it  is  much 
more  pronounced  in  fetal  than  in  adult  life.  It  has  been  shown, 
however,  that  it  is  formed  quite  independently  of  and  posterior 
to  the  true  Rathke's  pouch  (Killian),  though  what  its  signifi- 
cance ma\-  be  is  still  uncertain. 

The  tonsils  are  formed  from  the  epithelium  of  the  lower 
part  of  the  second  branclral  groove.  At  about  the  fou/th 
month  solid  buds  begin  to  grow  from  the  epithelium  into 
the  subjacent  mesenchyme,  and  depressions  appear  on  the 
surface  of  this  region.  Later  the  buds  become  hollow  by 
a  cornification  of  their  central  cells,  and  open  upon  the 
floor  of  the  depressions  which  represent  the  crypts  of  the 
tonsil.  In  the  mean  time  lymphocytes,  concerning  whose 
origin  there  is  a  difference  of  opinion,  collect  in  the  sub- 
jacent mesenchyme  and  eventually  aggregate  to  form 
lymphatic  follicles  in  close  relation  with  the  buds. 
Whether  the  lymphocytes  wander  out  from  the  blood  into 
the  mesenchyme  or  are  derived  directly  from  the  epithe- 
lial cells  is  the  question  at  is.sue. 

The  tonsil  may  grow  to  a  size  sufficient  to  fill  up  com- 
pletely the  depression  in  which  it  forms,  but  not  infre- 
(luentl)'  a  marked  depression,  the  jossa  supratousillaris, 
exists  above  it  and  represents  a  portion  of  the  original 
second  branchial  furrow.  Another  portion  of  the  same 
fur*-ow  is  represented  by  a  more  or  less  prominent  de- 
pression situated  pcsteriorly  to  the  opening  of  the  Eusta- 
chian tube  on  each  side  and  known  as  the  groove  of 
Rosenmiiller. 

The  Development  of  the  Branchial  Epithelial  Bodies. — 
These  are  structures  which  arise  either  as  thickenings  or 
as  outpouchings  of  the  epithelium  lining  the  lower  portions 


THF    IIKANCHIAI.    EriTIIELIAI.    UODIKS. 


3«3 


of  the  inner  branchial  furrows.  Five  pairs  of  these  struc- 
tures are  developed  and,  in  addition,  there  is  a  single  un- 
paired median  body.  This  last  makes  its  appearance  in 
embryos  of  about  3  mm-,  and  gives  rise  to  the  major  por- 
tion of  the  thyreoid  body.  It  is  situated  immediately  be- 
hind tlie  anterior  portion  of  the  tongue,  at  the  apex  of  the 
groove  between  t' is  and  the  posterior  portion,  and  is  first 
a  slight  pouch-like  depression.  As  it  deepens,  its  extrem- 
ity iiecomes  bilobed,  and  after  the  embryo  has  reached  a 
length  of  6  nun.  it  becomes  completely  separated  from  the 


Fir,     165      REcoNsTKtcTioNS  OF  THE    Bkanciuau    K1MTHE1.1AU    Bodies 

OK    KmBKVOS   ok    (.1)     14    MM.    AND  (H)    26    MM. 

an    Aorta;   ////,  lateral  tlivre..i(l;   />/(.  pliarynx;   /»//i'  and    /'//i\  parathy- 
'    reoids;  ///,  tliymud ;  //;r,  thymus;  vc,  vena  cava  superior,      (lour'uux 
and   r<T(/««.) 

floor  of  the  pharynx.  The  point  of  its  original  origin  is, 
however,  permanently  marked  bya  circular  depression,  the 
jomtnen  ccccum  (Fig.  i6i,  k).  Later  the  bilobed  body 
migrates  down  the  neck  and  becomes  a  solid  transversely 
elongated  mass  (Fig.  165,  th),  into  the  substance  of  which 
trabeculae  of  connective  tissue  extend,  dividing  it  into  a 
network  of  anastomosing  cords  which  later  divide  trans- 
versely to  form  follicles.  When  the  embryo  ha^  reached 
a  length  of  2.6  cm.,  a  cylindrical  outgrowth  arises  from  the 


r.i 


!i 


Hi  n 


3»4 


TlIK    I)KVKt.OrMr.NT    Ol'    TIIK    HUMAN    IIODV. 


anterior  s'  rface  of  the  mass,  usually  a  little  to  the  left  of 
the  median  line,  and  extends  up  the  neck  a  varying  dis- 
tance, forming,  when  it  persists  until  adult  life,  the  so- 
called  pyramid  of  the  thyreoid  body. 

This  account  of  the  pyramid  follows  the  statements  made  by 
recent  workers  on  the  question  (Tourneux  and  Verdun):  His 
has  claimed  that  it  is  the  remains  of  the  stalk  connecting  the 
thyreoid  with  the  floor  of  the  pharynx,  and  which  he  terms 
the  thyreo-glossal  duct. 

In  addition  to  this  median  structure,  one  of  the  pairs 
of  the  lateral  evaginations  also  take  part  in  the  formation 
of  the  thyreoid  body.  These  are  the  lateral  thyreoids 
(Fig.  165,  Ith),  and  they  arise  from  the  posterior  wall  of 
the  fourth  branchial  furrow,  in  embryos  of  about  8  mm. 
Separating  from  the  furrow,  they  migrate  backward  to 
fuse  in  embryos  of  about  16  mm.  with  the  posterior  surface 
of  the  lateral  portions  of  the  median  thyreoid.  They 
form,  however,  only  a  relatively  small  portion  of  the  entiie 
thyreoid  (Fig.  166,  thm  IV). 

Two  other  pairs  of  bodies  enter  into  intimate  relations 
with  the  thyreoid,  forming  what  have  been  termed  the 
parathyreoid  bodies  (Fig.  165,  pth^  and  ptlr).  One  of 
these  pairs  arises  as  a  thickening  of  the  anterior  wall  of 
the  fourth  branchial  groove  and  the  other  comes  from  the 
corresponding  wall  of  the  third  groove.  The  members  of 
the  former  pair,  after  separating  from  their  points  of  ori- 
gin, come  to  lie  on  the  dorsal  surface  of  the  lateral  portions 
of  the  thyreoid  body  (Fig.  1 66,  ptltm  I V )  in  close  proximity 
to  the  lateral  thyreoids,  while  the  latter,  passing  further 
backward,  come  to  rest  behind  the  lower  border  of  the 
thvreoid  (Fig.  166.  pthm  III).  The  cells  of  these  bodies  do 
not  become  divided  into  cords  by  the  ingrowth  of  connec- 
tive tissue  to  the  same  extent  as  those  of  the  thyreoids, 
nor  do  thev  become  separated  into  follicles,  so  tliat  the 


Tin;    IIUANCIIIAI.    KITIIIKIIAI.    HODIKS. 


3'5 


pthm  l\' 


I 


bodies  arc  readily  distiuKuishabU'  l)y  their  structure  from 

the  thyreoid. 

From  the  posterior  wall  of  the  third  branchial  groove  a 

pair    of    evaginations 
develop,     similar     to 
those    which    produce 
the  lateral   thyreoids. 
These  elongate  great- 
ly, and  growing  down- 
ward ventrally  t^  the 
thyreoid  and  separat- 
ing from  their  points 
of   origin,  come  to  lie 
below    the    thyreoids, 
forming     the    thymus 
gland  (Fig.    165,  thy). 
As  develofment    pro- 
ceeds      thi>y       pass 
furthei  backward  and 
come    eventually     to 
rest  upon  the  anterior 
surface    of    the    peri- 
cardium.    The  cavity 
wliich    they    at     first 
contain  is  early    -.>lit- 
erated  and  the  glands 
assume    a    lobed    ap- 
pearance and  become 
traversed     by    trabe- 
culae     of     connective 
tissue.    Lymphocytes, 

derived,  according  to  some  recent  observations,  directly 
from  the  epithelium  of  the  glands,  make  their  appearance 
and  gradually  increase  in  number  until  the  original  epi- 


I'li;      166.       TllYKlvOII.,  'lllVMl  s   .\M>    lU'i- 
TIIKUAI.       Hol'IKS       OK       A       Ni:\V  HOKN 

Child. 
/>.'/;»/  ///  and  t>tlim  IV,  raratliyrt'oids  ;   ></, 
lliyri-iiid;    ttim  III,    tliyimi^;    """   l^  < 
lateral  thyreoid.      ((Jruscliulj.) 


a 


-i 


i! 

I   I 

1  i 


V(> 


TIIK    l)KVKI,()l'MENT    OF    TIIK    HUMAN    l«OI>Y. 


tluliiil  ciUs  are  represented  only  hy  a  nutnher  of  peculiar 
spherical  structures,  consisting  of  cells  arranged  in  con- 
centric layers  and  known  as  Hussul's  corpuschs. 

The  glands  jnrsist  until  about  the  second  year  after 
hirth,  when  the\  undergo  a  dej^eneration  itito  a  mass  of 
fibrous  and  adipose  tissue. 


I'lc.   167.     DiACRAM  sHowivc  THK  okk.in  (h-  Till-:  Varkhs  Hkanchiai. 

Hi'iTiiKUiAu  Honiiis. 
llh.   Lateral    thyreoids;    /)/>,   postliraiuliiiil    hodies;    /^/i/'    and   /'///^   para- 

thyreoids ;'///,   tiiedian    lliyrenid;    //;),    thymus;   /    t(»    l\  ,   hraiu-liial 

grooves.      {Ktihii.) 

Finally,  a  pair  of  outgrowths  arise  from  the  floor  of  the 
pharynx  just  behind  the  fifth  branchial  arch,  in  the  region 
where  the  fifth  groove,  if  developed,  would  occur.  These 
post-brnuchial  bodies,  as  they  have  been  called,  usually 
undergo  degeneration  at  an  early  stage  and  disappear 
completely,  though  occasionally  they  persist  as  cystic 
structures  embedded  in  the  substance  of  the  thyreoid. 


4 


THE   (KSOPIIAOL'S. 


V7 


The  relation  ..f  these  various  structures  to  the  bruiulual 

« JI^.s  Jsthowu  by  .he  annexe.)  diuKran.  (K.K-  .67) ;  and  frou, 

it    it  will  be  seen  that  the  bodies  derived  from  the  third  aiul 

ou  th  Rcoves  are  serially  equivalent.     .^'•'"M.-rative  enibry 

,1  gv  inakes  this  fact  still  more  evident,  since,  in  the  lower  ^vr 

Sate" each  branchial  groove  -^-^^^^^^^ ^^l^^'l^'ZV^^^ 
the  thvmus  gland.  The  terminology  used  alx.Ne  for  the  xan  s 
bod  es  is  that  generallv  applied  to  the  mammalian  organs,  ut 
U  would  be  betfer.  for  the  sake  of  -•"M>ar.son  wUh  mher  ve^^^^^^ 
brates  to  adapt  the  nomenclature  proi>..sc-(l  b>  (.r.siliul!.  w  10 
t  Jmi 'e-  ch  lat  ral  thvreoid  a  thvmus  IV.  while  each  thymus  lobe 
"thCmu  I.  Similarlv  the  parathyreoids  are  termed  pani- 
Ihvmus  III  and  IV,  the  term  thyreoid  \v:m^  Inmted  to  the 
median  thyreoid. 

The  Muscuhiturc  oj  tin  /'//./n» v.  -  The  pharynx  dilTers 
from  other  portions  of  the  archenteron  in  the  fact  that  its 
walls  arc  furnished  .vitli  voluntary  nuiscles,  the  principal 
(,f  which  are  the  constrictors  and  the  stylo-pharynKcus. 
This  peculiarity  arises  from  the  relations  of  the  pharynx 
to  the  branchial  arches.     It  has  been  seen  that  in  the 
hi^dier  mammalia  the  dorsal  ends  of  the  third,  fourth,  and 
fifth  branchial  cartilages  disappear ;  the  muscles  oni;inally 
associated  with  these  structures  persist,  however,  and  Kive 
rise  to  the  muscles  of  the  pharynx,  which  consetiuently  are 
innervated  bv  the  ninth  and  tenth  nerves. 

The  Development  of  the  (Esophagus.-  I'roin  the  ventral 
side  of  the  lower  portion  of  the  pharynx  an  evai,nnatum 
develops  at  an  early  staj;e  which  is  destined  to  Rive  rise  to 
the  organs  of  respiration;  the  development  of  this  may, 
however,  be  conveniently  postponed  to  a  later  chapter 

(Chap.  XII).  .         .  ,, 

The  a^sophasus  is  at  first  a  very  short  portion  of  the 
archenteron  (Fig.  168,  A),  but  as  the  heart  and  diaphragm 
recede  into  the  thorax  it  elongates  (Fik.  i<'>B,  B)  until  it 
eventually  forms  a  considerable  portion  of  the  digestive 
tract      Its  endodermal  lining,  like  that  of  the  rest  of  the 


I 
.ttl 


!i 


Im 


1?!! 


318 


THE    PF-VELOPMENT   CF   THE    HUMAN    BODY. 


digestive  tract  except  the  pharynx,  is  surrounded  by 
splanchnic  mesoderm  whose  cells  become  converted  into 
non-striated  muscular  tissue,  which  by  the  fourth  month 
has  separated  into  an  inner  circular  and  an  outer  longitudi- 
nal layer. 


l'"[G.    168. —  Keconstkix'Tions  ok  thk    Dk.estive   Tract  of    Hmbrvos 

or  (.Ij   4.2  MM.  AM)  (/>')   5  M,M. 

all,  AUaiitois;  (7,  cldiica;  /,  luiii; ;  /;,  liver;  Kf>,  Ratlikc's  ])()ucli;  .S,  stonuicli; 
/,  tongue;  //(,  tliyrtMiid  body;  W'J,  W'olfl'iaii  duct ;  ;■,  yolk-stalk. — (His.) 


The  Development  of  the  Stomach  and  Intestines. —  By 

tlie  time  the  embryo  has  reached  a  length  of  about  3  mm. 
its  constriction  from  the  yolk-sac  has  proceeded  so  far  that 
a  portion  of  the  digestive  tract  anterior  to  the  yolk-sac 
can  bo  recognized  as  tlie  stomach  and  a  portion  posterior 
as  U".'  intestine.     At  first  the  stomach  is  a  simple  spindle- 


THE    STOMACH. 


319 


shaped  enlargement  (Fig.  168)  and  the  intestine  a  tube 
without  any  coils  or  bends,  but  since  in  later  stages  the 
intestine  grows  much  more  rapidly  in  length  than  the  ab- 
dominal cavity,  a  coiling  of  the  intestine  becomes  neces- 
sary. 

The  elongation  of  the  stomach  early  produces  changes 
in  its  position,  its  lower  end  bending  over  toward  the  right, 
while  its  upper  end,  owing  to  the  development  of  the  liver, 
is  forced  somewhat  toward  the  left.  At  the  same  time  the 
entire  organ  undergoes  a  rotation  about  its  longitudinal 
axis  through  nearly  90  degrees,  so  that,  as  the  result  of  the 
combination  of  these  two  changes,  what  was  originally  its 
ventral  border  becomes  its  lesser  curvature  and  what  was 
originally  its  left  surface  becomes  its  ventral  surface. 

Hence  it  is  that  the  left  pneuniogastric  nerve  passes  over  the 
vetitral  and  the  right  over  the  dorsal  surface  of  the  stomach  in 
the  adult. 

In  the  mean  time  the  elongation  of  the  oesophagus  has 
carried  the  stomach  further  away  from  the  lower  end  of 
the  pharynx,  and  from  being  spindle-shaped  it  has  become 
more  pyriform,  as  in  the  adult. 

The  growth  of  the  intestine  results  in  its  being  thrown 
into  a  loop  opposite  the  point  where  the  yolk-stalk  is  still 
connected  with  it,  the  loop  projecting  ventrally  into  the 
portionof  the  coelomic  cavity  which  is  contained  within  the 
umbilical  cord,  and  being  placed  so  that  its  upper  limb 
lies  to  the  right  of  the  lower  one.  I'pon  the  latter  a  slight 
pouch-like  lateral  outgrowth  appears  which  is  the  begin- 
ning of  the  ccecum  and  marks  the  line  of  union  of  the  fu- 
ture small  and  large  intestine.  The  sn  all  intestine,  con- 
tinuing to  lengthen  more  rapidly  than  the  large,  assumes  a 
sinuous  course  (iMg.  \Ch))  in  which  it  is  possible  to  recog- 
nize six  primary  coils  which  may  be  recognized  until  ad- 


320 


THE    DEVELOPMENT    OF   THE    HUMAN    HODY. 


■I 


vanccd  stages  of  development  and  even  in  the  adult 
(Mall).  The  first  of  these  is  at  first  indistinguishable 
from  the  pyloric  portion  of  the  stomach  and  can  be  recog- 
nized as  the  duodenum  only  by  the  fact  that  it  has  con- 
nected with  it  the  ducts  of  the  liver  and  pancreas;  as  de- 
velopment proceeds,  however,  its  caliber  diminishes  and  it 
assumes  the  appearance  of  a  portion  of  the  intestine. 


t\ 


•li 


FlC.     16<).     "RKCdNSTKl  CTIO.N   OK    IvMBKVO   OF    20    MM. 

C,  Caecum;    A",  kiciney;   L,   liver;    S,  st(iin;tcli;    SC,  suprarenal    l)odies; 
II',  nicsoncplirus. — (Mall.) 


The  remaining  coils  elongate  rapidly  and  are  thrown 
into  numerous  secondary  coils,  all  of  which  are  still  con- 
tained within  the  cfelom  of  the  umbilical  cord  (Fig.  170). 
When  the  embryo  has  reached  a  length  of  about  40  mm. 
the  coils  rather  suddenly  return  to  the  abdominal  cavity, 


THE    INTESTINE. 


32 


and  now  the  caecum  is  thrown  over  toward  the  ri.Kht,  so 
that  it  comes  to  lie  immediately  beneath  the  liver  on  the 
right  side  of  the  abdominal  cavity,  a  position  which  it  re- 
tains until  about  the  fourth  month  after  birth  (Treves). 
The  portion  of  the  large  intestine  which  formerly  pro- 
jected into  the  umbilical  coelom  now  lies  transversely 
across  the  upper  part  of  the  abdomen,  crossing  in  front  of 
the  duodenum  and  having  the  remaining  portion  of  the 
small  intestine  below  it.     The  elongation  continuing,  the 


Fki   170- REcoNSTRrcTioN  of  the  Intestine  ok  an  Embryo  ok  19  mm. 
The  l-u.iREs  ON  THE  Intestine  Indicate  the  I'rimaky  Coils. 

secondary  coils  of  the  small  intestine  become  more  numer- 
ous and  the  lower  portio.  of  the  large  intestine  is  thrown 
into  a  loop  which  extends  transversely  across  the  lower 
part  of  the  abdominal  cavity  and  represents  the  sigmoid 
flexure  of  the  colon.  At  the  time  of  birth  this  portion  of 
the  large  intestine  is  relatively  much  longer  than  in  the 
adult,  amounting  to  nearly  half  the  entire  length  of  the 
colon  (Treves),  but  after  the  fourth  month  after  birth  a 
readjustment  of  ilie  relative  lengths  of  the  parts  of  the 
27 


322 


THE    DEVELOPMENT    OF    THE    HUMAN    BOOY. 


colon  occurs,  the  sigmoid  flexure  becoming  shorter  and 
the  rest  of  the  colon  proportionally  longer,  whereby  the 
caecum  is  pushed  downward  until  it  lies  in  the  right  iliac 
fossa,  the  ascendina  colon  being  thus  established. 


»i 


I'lc.    171. — Rei'rese.nt.xtion  ok  the    Coilincs  ok  the    Intestine  in 
THE    AmxT   Condition.     Tin;    Xu.mueks    indic.xte    the    Pkim.\ry 

Coils.— (.\/(j//.) 

When  this  condition  has  been  reached,  the  duodenum, 
after  passing  downward  for  a  short  distance  so  as  to  pass 
dorsally  to  the  transverse  colon,  bends  toward  the  left  and 
the  secondary  coils  derived  fiom  the  second  and  third 


11 


THE    INTESTINE. 


323 


I 


.     -1 


primary  coils  come  to  occupy  the  left  upper  poition  of  ihe 
abdom..ial  cavity.  Those  from  the  fourth  primary  coil 
pass  across  the  middle  line  and  occupy  the  right  upper 
part  of  the  abdomen,  those  from  the  hfth  cross  back  again 
to  the  left  lumbar  and  iliac  regions,  and  those  of  the  sixth 
take  possession  of  the  false  pelvis  and  the  right  iliac  region 

(Fig-  171)- 

Slight  variations  from  this  arrangement  are  not  infrequent, 
but  it  occurs  with  sufficietU  frequency  to  be  regarded  as  Ihe 
normal.  A  failure  in  the  readjustmeiU  of  the  relative  lengths  ol 
the  difTerent  parts  of  the  colon  mav  also  oc  :asionally  occur,  in 
which  case  the  cscum  will  retain  its  emliryonic  position  beneath 
the  liver. 

The  yolk-stalk  is  continuous  with  the  intestine  at  the 
extremity  of  the  loop  which  extends  out  into  the  umbilical 
coelom,  and  when  the  primary  coils 
become  apparent  its  point  of  attach- 
ment lies  in  the  region  of  the  sixth 
coil.     As  a  rule,  the  caliber  of  the 
stalk  does  not  increase  proportionally 
with  that  of  the  intestine  and  even- 
tually its   embryonic  portion   disap- 
pears completely.  Occasionally,  how- 
ever, this  portion  of  it  does  paraike 
of  the  increase  in  si/e  which  occurs  in 
the  intestine,   and  it  forms  a  blind 
pouch  of   varying  Iciigth,  known  as 
Meckel s  diverticulum  (see  p.  135). 

The  ccECum  has  been  seen  to  arise  as  a  lateral  outgrowth 
at  a  time  when  the  intestine  is  first  drawn  out  into  the 
umbilicus.  During  subsecjucnt  development  it  continues 
to  increase  in  size,  until  it  forms  a  conical  i)ouch  arising 
from  tiie  colon  just  where  it  is  joined  by  the  small  intestine. 
The  enlargement  of  its  terminal  portion  does    'ot    keep 


Imc.    172.    -C/KciM    OK 

I'^MBKVO   OK    10.2  CM. 

f,  Colon;  i,  ileum. 


324 


THE    DEVELOPMENT   OF   THE    HUMAN    BODY. 


pace,  however,  with  that  of  the  portion  nearest  the  intes- 
tine, hut  it  becomes  gradually  more  and  more  distin- 
guished and  gives  rise  to  the  vermiform  appendix.  At 
birth  the  original  conical  form  of  the  entire  outgrowth 
is  still  distinguishable,  though  it  is  more  properly  de- 
scribed as  funnel-shaped,  but  later  the  proximal  part,  con- 
tinuing to  increase  in  diameter  at  the  same  rate  as  the 
colon,  becomes  sliarply  separated  from  the  appendix, 
forming  the  'I'O'cum  of  adult  anatomy. 


Viv,.    17.?.— RkconsTkiction'  of  .\   I'oktio.v  of   tiik    I.ntkstine  of  .\n 
Hmbrv(,  of  28  MM.,  sHOwiNi;  THE  I,o\<;iTri>i\.\L  Folds  from  v\hich 

THR    \'ILLI    ARE    Im)RMEI>.  — (/^i  »).V.) 


Up  to  the  time  when  the  embryo  has  reached  a  length  of 
14  mm.,  the  inner  surface  of  the  intestine  is  quite  smooth, 
but  when  a  length  of  19  nun.  has  been  reached,  the  mu- 
cous membrane  of  the  upper  portion  becomes  thrown  into 
longitudinal  folds,  and  later  these  make  their  appearance 
throughout  its  entire  length  fFig.  173).  T,ater,  in  em- 
bryos of  60  mm.,  these  folds  break  up  into  numbers  of 
conical  processes,   the   villi,   which   increase  in  number 


THK    LIVER. 


325 


with  the  devdopint-nt  (^f  the  inleslinc.  the  new  vilU  ap- 
pearing in  the  intervals  between  those  already  presetit. 

A  rcnarkable  phencuK-non  has  recently  been  described  as 
occurring  in  the  hu.denu.n  <.f  end)ry.)S..f  about  .2.5  nun.  It 
consis  s  in  a  rapid  Rrowth  in  the  ihick.iess  of  the  nmcous  meni- 
b  ne  w leeln^-  the  hunen  of  the  intestine  innned.ately  below 
he'ienU  ..f  the  hepatic  and  pancreatic  ducts  --"- 8^;^  b 
reduced  in  size  and  is  hnally  completelv  obliterate  1.  his 
".ndilion  persists  until  the  embryo  has  reached  ^^"S  '^  4-3 
nun  when  the  lumen  again  appears  ( I andler).  1  his  process  is 
rnteresting  in  connection  with  the  occasional  ..ccurrence  in 
new  born  children  of  an  atresia  of  the  duodenum. 

The  Development  of  the  Liver.-The  liver  makes  its 
appeararxe  in  embryos  of  about  3  mm.  as  a  longituchnal 
groove  upon  the  ventral  surface  of  the  archenteron  just 
below  the  stomach  and  between  it  and  the  umbilicus. 
The  endodermal  cells  lining  the  anterior  portion  of  the 
groove  early  undergo  a  rapid  proliferation,  and  form  a 
solid  mass  which  projects  ventrally  into  the  substance  of  a 
horizontal  shelf,  the  septum  transversum   (see  p.  33^')- 
attached  to  the  ventral  wall  of  the  body.     This  solid  mass 
(Fig   1 74,  L)  forms  the  beginning  of  the  liver  proper,  while 
the  lower  portion  of  the  groove,  which  remains  hollow, 
represents  the  future  gall-bladder  (Fig    174.  B).     Con- 
strictions appearing  between  the  intestine  and  both  the 
hepatic  and  cystic  portions  of  the  organ  gradually  separate 
these  from  the  intestine,  until  they  are  united  to  it  only 
by  a  stalk  which  represents  the  ductus  commums  cholcdo- 

chus  (Fig.  174). 

The  further  development  of  the  liver,  so  far  as  its  exter- 
nal form  is  concerned,  consists  in  the  rapid  enlargement  of 
the  hepatic  portion  until  it  occupiei-'  the  greater  part  of  the 
upper  half  of  the  abdominal  cavity,  its  ventral  edge  ex- 
tending as  far  down  as  the  umbilicus.  In  the  rabbit  its 
substance  becomes  divided  into  four  lobes  corresponding 


326 


THK    UEVELOI'MENT    OF    THE    HUMAN    B01)V. 


I    I 


i      ' 


to  the  four  veins,  unil>ilical  and  omphalo- mesenteric, 
which  traverse  it,  and  the  same  condition  occurs  in  the 
human  embryo,  althoujjh  the  lobes  are  not  so  clearly  indi- 
cated upon  the  surface  as  in  the  rabbit.  The  two  om- 
phalo-mesenteric  lobes  are  in  close  apposition  and  may 
almost  t)e  regarded  as  one,  a  median  ventral  lobe  which 
embraces  the  ductus  venosus  (Fig,  i"',  B,  DV),  while 
the  umbilical  lobes  are  more  lateral  and  dorsal  and  repre- 


%Ca->" 


Fir..    174.    -Reco.vstructio.ns  of  thk   Livkk   ( )itc.ro\vths  ok   R.xbbit 

IV.MBRVOS   OV    (A)    5    MM.    A.MJ    ( />' j    OF    8    MM. 

/>,  Gull-h1;i(lclcr;  (/,  diKKk-miiii ;  I)\',  ductus  vctiosus;  /.,  liver;  />»«,  ventral 
pancrt-as;  rL,  right  lobe  of  the  liver;  .S,  stouiaeh.     (llatunun.) 

sent  the  right  (rL)  and  left  lobes  of  the  adult  liver.  The 
remaining  definitive  lobes,  the  spigelian,  quadrate  and 
caudate,  are  of  later  formation,  the  first  two  standing  in 
relation  to  the  vessels  which  cross  the  lower  surface  of  tlic 
liver,  while  the  caudate  is  formed  by  a  portion  of  the  right 
lobe  which  ari..LS  across  the  upper  part  of  tlie  ductus 
venosus. 

The  ductus  communis  cho'edochus  is  at  first  wide  and 


TUT.    LIVER. 


327 


short,  and  near  its  proximal  end  gWcs  rise  to  a  small  out- 
growth on  each  side,  one  of  which  becomes  the  ventral 
nancreas  (KiR.  i7«,  B.  /"")•  Later  it  elongates  and  be- 
comes more  slender,  and  the  gall-bladder  is  constricted  otT 
from  it,  the  connecting  stalk  becoming  the  cystic  dud. 
The  hepatic  ducts  are  apparently  developed  from  the 
liver  substance  and  are  relatively  late  in  appearing. 

Shortly  after  the  hepatic  portion  has  been  differen- 
tiated its  substance  becomes  permeated   by   numerous 


Fu:.   175.     Tkansveksk  Skction  THK..r.;H  the  Livick  of  an   Umhkvo 

OK  I'oiK  Months. 
n,,  Intestine-;  /,  liver;  II',  WoltVuiu  Ixuly.     (Tohll  and  /urknkamll.) 

blood-vessels,  and  so  divided  into  numerous  anastomosing 
trabeculae  (Fig.  175).  These  are  at  first  irregular  in  size 
and  shape,  but  later  they  become  more  slender  and  more 
regularly  cyhndrical.fonning  what  have  been  termed  the 
hepatic  cylinders.  In  the  center  of  each  cylinder,  where 
the  cells  which  form  it  meet  together,  a  tine  canal  appears, 
the  beginning  of  a  hilc  capillary,  the  cylinders  thus  be- 
coming converted  into  tubes  with  fine  lumina.     This  oc- 


328 


THE    UKVELOI'MENT    OF    THE    HUMAN    IIODV. 


curs  at  about  the  fourth  week  of  development  ami  at  this 
time  a  cross-section  of  a  cylinder  shows  it  to  he  composed 
of  about  *',ree  or  four  hepatic  cells  (Imr.  176,  A),  anions 
which  arc  to  be  seen  groups  of  smaller  cells  (e)  which  are 
erythrocytes,  the  liver  havinj;  assumed  by  this  time  its 
haematopoietic  function  (st  -  n.  244).  This  condition  of 
affairs  persists  unt'l  birth.,  I  later  the  cylinders  undergo 
an  elongation,  the  cells  of  which  they  are  composed  slip- 


Fn;.    176.     Tkansvkksi:  Sf.ctions  ok  Portions  or  tmk  I.ivkk  ok  (.1) 
A  Fetis  ok  Six  Months  ani>  (H)  a  Ciiim>  ok  Foir  Years. 

crytlirocytc;    he,    heinitic    cylinder. -(7"('/<//    and 
/.tickt'ikiindl.) 


be,    Bile    capilhiry; 


ping  over  one  another  apparently,  so  that  the  cylinders 
become  thinner  as  well  as  longer  and  show  for  the  most 
part  only  two  cells  in  a  transverse  section  (Kig.  176,  B); 
and  in  still  later  periods  the  two  cells,  instead  of  lying 
opposite  one  another,  may  alternate,  so  that  the  cylinders 
become  even  more  slender. 

The  bile  capillaries  seem  to  make  their  appearance  first 
in  cylinders  which  lie  in  close  relation  to  branches  of  the 


TIIK    !  IVER. 


329 


portal  vein  (I-i^'.  .77)  and  thence  extend  throughout  the 
ncii^hborins;  evUnders.  anastomosiuK'  with  capillanes  de- 
velopinK  in  relation  to  neishhorin-  portal  branches.  x\s 
the  extension  so  proceeds  the  older  capillaries  c.mtnu.e  to 
enlarge  and  later  become  transfortned  into  hlcdiutsilni^. 
,^-  C)  the  cells  of  ija- cylinders  in  which  these  capillaries 
were  situated  becoming  converted  into  the  epithelial  lining 

of  the  ducts.  .     .  . 

The  lobules,  which  form  so  characteristic  a  feature  ot 

the  adult  liver,  are  late  in  appearin.u.  not  beini;  fully  de- 


Fi. 


177       IMKCTKU  IM..U   Capiu.akiES  or  I'l..    I':mhkv..s  OV  {\)  8  CM.. 
(li)   U.  CM.,  ANi.  (f)  OK  Ai.iXT  Vi'..     (Ilnulnckum.) 


vdoped  until  some  time  after  birth      They  depend  upon 
the  relative  arraujrcment  of  the  branches  of  the  portal  anc 
hepatic  veins;  these  at  first  occupy  distinct  territories  of 
the  liver  sul)stance,  beiuR  separated  from  one  another  by 
practicallv  the  entire  thickness  of  the  liver,  althoujrh  cf 
course  connected  bv  the  capillaries  which  lie  between  the 
hepatic  cvlinders.     During  development  the  two  sets  of 
branches  "extend  more  deeply  into  the  liver  substance, 
each  invading  the  territory  of  the  other,  but  tlicy  can 
readily  be  distinguished  from  one  another  by  the  fact  that 
the  portal  branches  are  enclosed  within  a  sheath  of  con- 
28 


330 


THE    DKVKLOI'MKNT   Ol"    THK    HUMAN    IIODV. 


ntctivf  tissue  ((Uisson's  capsule)  wliicli  is  lacking  to  the 
hepatic  vessels.    At  about  the  time  of  birth  tlie  branches  of 
the  hepatic  vins  give  off  at  intervals  bunches  of  terminal 
vessels,  around  which  branches  of  the  portal  vein  arrange 
themselves,  the  liver  tissue  becoming  divided  up  into  a 
number  of  areas  which  may  be  termed  lupatic  islands, 
each  of  which    is    surrounded   by  a  number  of  portal 
i)ranches  and  contains  numerous  dichotomously  branch- 
ing hepatic  terminals.     Later  the  portal  branches  sink 
into  the  substance  of  the  islands  which    thus    become 
lol)ed,  and  tinally  the  sinking  in  extends  so  far  that  the 
original  island  becomes  separated  into  a  number  of  smalUr 
areas  or  lobules,  each  containing,  as  a  rule,  a  single  hepatic 
terminal  (the  intralobular  vein)  and  being  surrounded  by  a 
number  of  portal  terminals  (interlobular  veins),  the  two 
systems  being  united  by  the  capillaries  which  separate  the 
cylinders  contained  within  the  area.     The  lobules  are  at 
first  very  small,  but  later  they  increase  in  size  by  the  ex- 
tension of  the  hepatic  cylinders. 

Kretiucntlv  in  the  human  liver  lobules  are  to  be  found  con- 
taining two  intralobular  veins,  a  condition  which  results  from 
an  iniix-rfect  subdivision  of  a  lobe  of  the  original  hepatic  island. 

The  liver  early  assumes  a  relatively  large  size,  its  weight 
at  one  time  being  equal  to  that  of  the  rest  of  the  body,  and 
though  in  later  embryonic  stages  its  relative  size  dimin- 
ishes,  yet  at  birth  it  is  still  a  voluminous  organ,  occupying 
the  greater  portion  of  the  upper  half  of  the  abdominal 
cavity  and  extending  far  over  into  the  left  liypochon- 
drium.  Just  after  birth  there  is,  however,  a  cessation  of 
growth,  and  the  subsecjuent  increase  proceeds  at  a  much 
slower  rate  than  that  of  the  rest  of  the  body,  so  that  its  rela- 
tive size  becomes  still  more  diminished  (see  Chap.  XVI). 
The  cessation  of  growth  affects  principally  the  left  lobe 


THE    I'ANCKEAS. 


331 


' 


and  (leiutids  upcn  an  actual  d-i^'ncration  of  portions  of 
the  liver  tissue,  the  cells  disappearing  completely,  while 
the  ducts  and  l.loo<l  v.      Is  originally  present  persist,  the 
former  constituting  the  vasa  ahenantia  of  adult  anatomy. 
These  are  usually  espe- 
cially noticeable    at  the 
left  edge  of  the  liver,  be- 
tween   the   folds  of   the 
left  lateral  ligament,  but 
they  may  also  be  found 
alonjj  the  line  of  the  vena 
cava,    around    the    gall- 
1  ^ladder,  and  in  the  re- 
j^ion  of  the  left  longitudi- 
nal fissure. 

The    Development    of 
the  Pancreas.-  The  pan- 
creas arises  a  little  later 
than  the  liver,  as  three 
separate  outgrowths,  one 
from   the  dorsal  surface 
of   the  duodenum   (Fig. 
1 78,  DP)  almost  opposite 
the  liver  outgrowth,  and 
one  on  each  side  from  the 
lower  part  of   the  com- 
mon bile-duct.      Of   the 
latter    outgrowths,   that 
upon  the  left  side  (Vps) 
early  begins  to  degener- 
ate and  completely  disappears,  while  that  of  the  right 
side  {Vpd)  continues  its  development  to  form  what  has 
been  termed  the  ventral  pancreas.     Both  this  and  the 
dorsal  pancreas  continue  to  elongate,  the  latter  lying  to 


Fig.  178.-~REcoNSTRrcTi(>N  of  the 
Pancreatic  Oitorowtiis  ok  an 
Embryo  ok  7.5  mm. 

I),  Duodenum;  Ih,  ductus  coninuinis 
choledochus ;  DP,  dorsal  pancreas; 
V/x/  and  V'/'.f,  right  and  left  ven- 
tral pancreas.— (//f//>.) 


332 


THE    DEVELOPMENT   OF   THE    HUMAN    HODY. 


I     I 


i    ; 


:   i 


the  left  of  the  portal  vein,  while  the  former,  at  first  situ- 
ated to  the  right  of  the  vein,  later  grows  across  its  ventral 
surface  so  as  to  come  into  contact  with  the  dorsal  gland, 
with  which  it  fuses  so  intimately  that  no  separation  line 
can  be  distinguished.  The  body  and  tail  of  the  adult 
pancreas  represent  the  original  dorsal  outgrowth,  while 
the  right  ventral  pancreas  becomes  the  he  i '. 

Both  the  dorsal  and  ventral  outgrowths  early  become 
lobed,  and  the  lobes  becoming  secondarily  lobed  and  this 
lobation  repeating  itself  several  times,  the  compound 
tubular  structure  of  the  adult  gland  is  acquired,  the  very 
numerous  terminal  lobules  becoming  the  secreting  acini, 
while  the  remaining  portions  become  the  ducts.  Of  the 
principal  ducts,  there  are  at  first  two;  that  of  the  dorsal 
pancreas,  the  duct  oj  Santorini,  opens  into  the  duodenum 
on  its  dorsal  surface,  while  that  of  the  ventral  outgrowth, 
the  duct  of  Wirsung,  opens  into  the  ductus  communis  chol- 
edochus.  When  the  fusion  of  the  two  portions  of  the 
gland  occurs,  an  anastomosis  of  branches  of  the  two  ducts 
develops  and  the  terminal  portion  of  the  duct  of  Santorini 
usually  degenerates,  so  that  the  secretion  of  the  entire 
gland  empties  into  the  common  bile-duct  through  the  duct 

of  Wirsung. 

In  the  connective  tissue  which  separates  the  lobules  of 
the  gland  groups  of  cells,  arranged  so  as  to  form  anasto- 
mosing trabecular,  occur.  These  appear  to  have  no  con- 
nection with  the  ducts  of  the  gland,  and  form  what  are 
termed  the  areas  oj  Langerhans.  They  seem  to  arise  by 
the  separation  off  of  portions  of  the  acini,  but  what  their 
later  history  and  function  may  be  is  as  yet  uncertain. 


LITFRATURE. 

J.  M.  HivKkv  ;  "On  the  rk-veloptncnt  of  the  Villi  of  the  Human  Intestine," 
Amit.  Anzcigcr,  xvi,  1900. 


LITERATURE. 


333 


J 

J- 

K 

J 

.1. 

K. 

W. 

\V. 
F. 

G. 

A. 

v. 

J- 

c. 

A. 

J- 
C. 

F 


BracheT:    "  Rccl.crcl.es  sur  Ic  dcveloppement  du  pancreas  et  du  fnic," 

H.    C.UEviTz:  "Beitrage    zur    Kntwicklungsgcschichte    der   Spe.cl.el- 

drusen  "  Archir  fur  Armt.  und  Physiol,  Anai.  Ahth.,  1885. 

Oroscufk:  "Ucbcr  das  Vorkomn.cn   eincs   Thymusscgmcntcs  dcr 

vierten  Kienicntasche  beim  Menschcn,"  Anat.  Ameiger,  xvii.  1900. 

A   H  XMM  XR    ••  Hinige  Plattcnnu.delle  zur  Belcuchtung  der  fruheren  e.ii- 

■bryonaU.el)erentwicklung.'' /Ircfc.r /«r  .lH«/  ««<//'/,>■«.>/..  .1  «'»'■   IW/... 

A    H.xmm.ar:  "Notiz  tiber  die  Kntwickclung  der  Zunge  und  der  Mund- 
spcichcldruscn  bcim  Menschcn."  ^1  nat.  A  nzeigcr,  xix,  1901 . 
HEI.UV  "Zur    Untwickclungsgeschichte    der    Pancreasanlagen    unrt 
Duodenalpapillen  des  Menschcn,"  Archiv  jiir  mikrosk.  Anat.,  LVI,  1900. 
V    Heni.RICKSOn:  "The   Development  of  the   Bilc-capillanes  as  re- 
vealed byOolgi's  Method,"  Johns  Hopkins  Hospital  Hullctin,  1898. 
His-  "Anatomic  menschlicher  Kmbryonen."  Leipzig.  1882-1886. 
KEiBEU:  "/Cur  Kntvvickclungsgcscl.ichtc  des  mcnschlichen  Un-eenUal- 
appuratus,"  Archiv  jitr  Anat.  und  I'hysioL,  Anat.  Ahth.,  1896. 
Kii,ui.\N.  "Ueber  die  Bursa  und  Tonsilla  pharyngea."  Morphol.  Jahr- 

huch,  XIV,  1888.  _,    r-    .      •  z. 

Kohn:  "Die  Epithelkorpcrchen,"  Ergebnissc  der  Anat.  und  Entwick- 

liingsi^c.'.cb.,  IX,  1899. 
P    M.\LU:  "Ueber  die   Entwickclung  des  menschlichen   Darnies  und 

seiner  I.age  beim  Ervvachsencn,"  Archiv  jiir  Anat.  und  Physiol.  Anat.. 

Ahth.  Supplement,  1897. 
r.  Meckel:  "  Bildungsgeschichte  des  Darmkanals  der  Saugethicre  und 

namentlich  des  Mens<.hen,"  Archiv  jiir  Aiuit.  und  Physiol.,  ill,  1817. 
rose:  "Ueber  die  Entwieklung  der  Ziihne  des  Menschcn,"  Archiv  jiir 

mikrosk.  Anat.,  xxxviil,  1891. 
S\v.\En:    '  Recherches  sur  Ic  developpement  du  foie,  du  tube  digestif, 

dc  I'arricrc-cavite  du  peritoine  et  du  niesenttre,"  Journ.  de  I'Anat.et 

dela  Phvsiol.,  xxxii,  1896,  and  xxxiii,  1897. 
T.wduer:  "Zur  Entwicklungsgeschichtc  des  mcnscliHchen  Duodenum 

infriilien  Embryonalstadien,"  .Morphol.  Jahrhurh,\xix,  1900. 
TouuT  .\Ni)  E.  Zi'CKERKANDL-  "  Ucber  die    Form  und    Tcxturvenin- 

dcrungen  dei  mcnschlichen  Leber  vvahrend  des  Wachsthums,"  Sitz- 

unfisher.  der  kais.  .\kad.  Wissensch.   IVien.,  Math.-Xaturuiss.   Classe, 

uxxii,  1875. 
TouRNEux  AND  P.  Verihtn:  "Sur  Ics  premiers  dcveloppements  dc  la 
Thyroidc,  du  Thymus  et  des  glandcs  parathyroidienncs  chez  I'humme," 

journ.  de  I'Amit.  et  de  la  Physiol.,  xxxiii,  1897. 
Treves:  "Lectures  on  the  .\natumy  of  the  Intestinal  Canal  and  Peri- 
toneum in  Man,"  British  .Medical  Journal,  I,  1885. 


u\ 


CHAPTER   XI. 

THE    DEVELOPMENT  OF    THE    PERICARDIUM 
AND  PLEURO-PERITONEUM,  THE  DIA- 
PHRAGM   AND    THE    SPLEEN. 

It  has  been  seen  (p.  248)  that  the  heart  makes  its  ap- 
pearance at  a  stage  when  the  greater  portion  of  the  ven- 
tral surface  to  the  intestine  is  still  open  to  the  yolk-sac. 
The  ventral  mesoderm  splits  to  form  the  somatic  and 
splanchnic  layers  and  the  heart  develops  as  a  fold  in  the 
latter  on  each  side  of  the  median  line,  projecting  into  the 
coelomic  cavity  enclosed  by  the  two  layers  (Fig.  126,  A). 
As  the  constriction  of  the  anterior  part  of  the  embryo 
proceeds,  the  two  heart  folds  are  brought  nearer  together 
and  later  meet,  so  that  the  heart  becomes  a  cylindrical 
structure  lying  in  the  median  line  of  the  body  and  is  sus- 
pended in  the  ccelom  by  a  ventral  band,  the  ventral  meso- 
cardium,  composed  of  two  layers  of  splanchnic  mesoderm 
which  extend  to  it  from  the  ventral  wall  of  the  body,  and 
by  a  similar  band,  the  dorsal  mesocardium,  which  unites 
it  with  the  splanchnic  mesoderm  surrounding  the  diges- 
tive tract.     The  ventral  mesocardium  soon  disappears 
(Fig.  126,  C)  and  the  dorsal  one  also  vanishes  somewhat 
later,  so  that  the  heart  comes  to  lie  freely  in  the  coelomic 
cavity,  except  for  the  connections  which  it  makes  with 
the   body-walls  by   the  vessels  which  enter   and   arise 

from  it. 

The  coelomic  cavity  of  the  embryo  does  not  at  first  com- 
municate with  the  extra  embryonic  coclom,  which  is 
formed  at  a  very  early  period  (see  p.  84),  but  later  when 

334 


THE    PERICARDIUM. 


335 


the  splitting  of  the  embryonic  mesoderm  talces  place  the 
two  cavities  become  continuous  behind  the  heart  but  not 
anteriorly,  since  the  ventral  wall  of  the  body  is  formed  m 
the  heart  region  before  the  union 
can  take  place      It  is  possible, 
therefore,  to  recognize  two  por- 
tions in  the  embryonic  ccelom,  an 
anterior  one,  the  parietal  cavity 
(His),  which  is  never  connected 
laterally  with  the  extra-embry- 
onic cavity,  and  a  posterior  one, 
the  trunk  cavity,  which  is  so  con- 
nected.    The  heart  is  situated  in 
the  parietal  cavity,  a  consider- 
ab'    r>ortion  of  which  is  destined 
t.    ,    ■::)me  the  pericardial  cavity. 
•     tee  the  parietal  cavity  lies 
i....diately  anterior  to  the  still 
wide  yolk-stalk,  as  may  be  seen 
from  the  position  of  the  heart  in 
the  embryo  shown  in  Fig.  42,  it 
is  bounded  posteriorly    by  the 
yolk-stalk.      This    boundary   is 
complete,  however,  only  in  the 
median   line,  the   cavity   being 
continuous  on  either  side  of  the 
yolk-stalk  with  the  trunk-cavity 
by  passages   which    have   been 
termed    the    recessus   parietales 
(Fig.  179,  Bp  and  Rca).     Pass- 
ing forward  toward  the  heart  in 

the  splanchnic  mesoderm  which  surrounds  the  yolk-stalk 
are  the  large  omphalo-mesenteric  veins,  one  on  either 
side,  and  these  shortly  become  so  large  as  to  bring  the 


Om 


Rca 


Fig.  179. — Reconstruction 
op  a  r.vbbit  p^mbryo  of 
Eir.HT  Days,  with  the 
Pericardial  Cavity  Laid 
Open. 

A,  Auricle;  Aob,  aortic  bulb; 
A.V.,  auriculo-ventricular 
communication;  Up,  ven- 
tral parietal  recess;  Om, 
omphalo  -  mesenteric  vein ; 
Pc,  pericardial  cavity ;  Rca, 
dorsal  pariptal  recess ;  sr, 
sinus venosus;  V',  ventricle. 
—(His.) 


!l 

f    ^1 


336 


THE    DEVELOPMENT   OF    THE    HUMAN    liODV. 


I 


t 

i      I 


ft       I 


splanchnic  mesoderm  i!i  which  they  lie  in  contact  with 
the  somatic  mesoderm  which  forms  the  lateral  wall  of 
each  recess.  Fusion  of  the  two  layers  of  mesoderm 
along  the  course  of  the  veins  now  takes  place,  and 
each  recess  thus  becomes  divided  into  two  parallel  pas- 
sages, which  ),4ve  been  termed  the  dorsal  (Fig.  i8o,  rpd) 
and  ventral  {rpv)  parietal  recesses.  Later  the  two  veins 
fuse  in  the  upper  portion  of  their  course  to  form  the  begin- 
ning of  the  sinus  venosus,  with  the  result  that  the  ventral 
recesses  become  closed  below  and  their  continuity  with  the 


Fi<;  180  -Transveksk  vSections  of  a  Rabdit  Hmbkyo  showinc  the 
Division  of  the  Parietal  Recesses  by  the  Omphalomesenteric 
Veins.  .  . 

rtm.  Amnion;  r/.,  parietal  recess;  rpd  and  r/.-r,  dorsal  and  ventral  divisions 
of  the  parietal  recess;  vom,  oinphalo-niesenleric  vein,     {hain.) 

trunk-cavity  is  interrupted,  so  that  they  form  two  blind 
pouches  extending  downward  a  short  distance  from  the 
ventral  portion  of  the  floor  of  the  parietal  cavity.  The 
dorsal  recesses,  however,  retain  their  continuity  with 
the  trunk-cavity  until  a  much  later  period. 

By  the  fusion  of  the  omphalomesenteric  veins  men- 
tioned above,  there  is  formed  a  thick  semilunar  fold  which 
projects  horizontally  into  the  coelom  from  the  ventral  wall 
of  the  body  and  forms  the  floor  of  the  ventral  part  of  the 
parietal  recess.     This  is  known  as  the  septuyn  iransvcr- 


L»    I 


THE    PERICAROIUM. 


337 


sum  and  besides  containing  the  anterior  portions  of  the 
omphalomesenteric  veins,  it  also  furnishes  a  passage  by 
which  the  ductus  Cuvieri.  formed  by  the  union  of  the 
jugular  and  cardinal  veins,  reaches  the  heart.  Its  dorsal 
edge  is  continuous  in  the  median  line  with  the  mesoderm 
surrounding  the  digestive  tract  just  opposite  the  region 


am 


Vic,      181     -RECONSTKrCTION    FROM    A     RABBIT     IvMBRVO    OF     NiNE     DaYS 
SHOWINC.   THE  SEPTI-M  TkANSVEKSUM    FROM    AbOVE. 

am    Amnion;  (J/,  iiuriclc;  ./r.  ductus  Cuvieri;  r(>,{,  dorsal  parietal  recess. - 

(Riiiii.) 

where  the  liver  outgrowth  will  form,  but  laterally  this 
edge  is  free  and  forms  the  ventral  walls  of  the  dorsal  parie- 
tal recess.  An  idea  of  the  relations  of  the  septum  at  this 
stage  may  be  obtained  from  Fig.  1 8 1 .  which  represents  the 
anterior  surface  of  the  septum,  together  with  the  related 
parts,  in  a  rabbit  embryo  of  nine  days. 


r 


338 


THE    DEVELOPMENT    OF   THE    HUMAN    HODY. 


1 1 

.Is 

l| 


1  1 


The  Separation  of  the  Pericardial  Cavity— The  septum    , 
trans versum  is  at  first  almost  horizontal,  but  later  it  be- 
comes decidedly  oblique  in  position,  a  change  associated 
with  the  backward  movement  of  the  heart.     As  the  clo- 
sure of  the  ventral  wall  of  the  body  extends  posteriorly 
the  ventral  edge  of  the  septum  gradually  slips  downward 
upon  it,  while  the  dorsal  edge  is  held  in  its  former  position 
by  its  attachment  to  the  wall  of  the  digestive  tract  and 
the  ductus  Cuvieri.     The  anterior  surface  of  the  septum 
thus  comes  to  look  ventrally  as  well  as  forward  and  the 
parietal  cavity,  having   taken   up   into  itself   the  bUnd 
pouches  which  represented  the  ventral  recesses,  comes  to 
lie  to  a  large  extent  ventral  to  the  posterior  recesses.    As 
may  be  seen  from  Fig.  181,  the  ductus  Cuvieri,  as  they 
bend  from  the  lateral  walls  of  the  body  into  the  free  edges 
of  the  septum,  form  a  marked  projection  which  diminishes 
considerably  the  opening  of  the  dorsal  recesses  into  the 
parietal  cavity.     In  later  stages  this  projection  increases 
and  from  its  dorsal  edge  a  fold,  which  may  be  regarded  as 
a  continuation  of  the  free  edge  of   the  septum,  projects 
into  the  upper  portions  of  the  recesses  and  eventually 
fuses  with  the  median  portion  of  the  septum  attached  to 
the  wall  of  the  gut.     In  this  way  the  parietal  cavity  be- 
comes a  completely  closed  sac,  and  is  henceforward  known 
as  the  pericardial  canity,  the  original  coelom  being  now 
divided  into  two  portions,  (i)  the  pericardial  and  (2)  the 
pleiiro- peritoneal  cavities,  the  latter  consisting  of  the  ab- 
dominal cd^om  together   with   the   two   dorsal  parietal 
recesses  which  \\a\Q  been  separated  from  the  pericardial 
(parietal )  cavity  and  are  destined  to  be  converted  into  the 
pleural  cavities. 

The  Formation  of  the  Diaphragm. — It  is  to  be  remem- 
bered that  the  attachment  of  the  transverse  septum  to  the 
ventral  wall  of  the  digestive  tract  is  opposite  the  point 


THE    DIAPHRAGM. 


339 


where  the  liver  outgrowth  develops.  When,  therefore, 
the  outgrowth  appears,  it  pushes  its  way  into  the  sub- 
stance of  the  septum,  which  thus  acquires  a  very  con- 
siderable thickness,  especially  toward  its  dorsal  edge,  and 
it  furthermore  becomes  differentiated  into  two  layers,  an 
upper  one,  which  forms  the  floor  of  the  ventral  portion  of 
the  pericardial  cavity  and  encloses  the  Cuvierian  ducts, 
and  a  lower  one  which  coi  <  lins  the  liver.  The  upper  layer 
is  comparatively  thin,  while  the  lower  forms  the  greater 


Vu:  182  -DiAC.RAMS  oK  (A)  a  Sa<:ittau  Section  op  an  Hmbrvo  shovv  n(. 
THE  Liver  Enclosed  within  the  Septum  Iransversum;  {H)  a 
Front \L  vSection  ok  the  Same;  (O  a  Frontau  Section  ok  a  Lateu 
Stac.e  when  the  Livek  has  Separated  from  the  Diaphrac.m. 

All  AllanK.is;  t7,  cloaca;  U,  diaphragm;  Li,  liver;  U,  sus,)onsory  li^atnent 
'  of  the  liver;  M,  mesi-nierv;  A/j;,  mesoKastruuii ;  I'c,  iwncardiuni , 
.S,  stomach;  .ST,  septum  transversiim;  U,  umbilicus. 

part  of  the  thickness  of  the  septum,  its  posterior  surface 
meeting  the  ventral  wall  of  the  abdomen  at  the  level  of 
the  anterior  margin  of  the  umbilicus  (Fig.  182,  A). 

In  later  stages  of  development  the  layer  containing  the 
liver  becomes  separated  from  the  upper  layer  by  two 
grooves  which,  appearing  at  the  sides  and  ventrally  imme- 
diately above  the  liver  (Fig.  182,  B),  gradually  deepen 
toward  tae  median  line  and  dorsally.     These  grooves  do 


340 


THE    DEVELOrMKNT    OK    THE    HUMAN    HOPV. 


not,  however,  quite  reach  the  median  line,  a  portion  of  the 
lower  layer  of  the  septum  being  left  in  this  region  as  a  fold, 
situated  in  the  sagittal  plane  of  the  body  and  attached 
above  to  the  posterior  surface  of  the  upper  layer  and  be- 
low to  the  anterior  surface  of  the  liver,  beyond  which  it  is 
continued  down  the  ventral  wall  of  the  abdomen  to  the 
umbilicus  (Fig.  182,  C,  Ls).     This  is  the  suspensory  liga- 
ment of  the  liver  of  adult  anatomy,  and  in  the  free  edge  of 
its  prolongation  down  the  ventral  wall  of  the  abdomen  the 
umbilical  vein  passes  to  the  under  surface  of  the  liver, 
while  the  free  edge  of  that  portion  which  lies  between  the 
liver  and  the  digestive  tract  contains  the  omphalo-mesen- 
teric  fportal)  vein,  the  common  bile-duct,  and  the  hepatic 
arterv.     The  diagram  given  in  Fig.  182  will,  it  is  hoped, 
make-  clear  the  mode  of  formation  and  the  relation  of  this 
fold,  which,  in  its  entirety,  constitutes  what  is  sometimes 
termed  the  ventral  mesentery. 

And  not  only  do  the  grooves  fail  to  unite  in  the  median 
line,  but  they  also  fail  to  completely  separate  the  liver 
from  the  upper  layer  of  the  septum  dorsally,  the  portion 
of  the  lower  layer  which  persists  in  this  region  forming 
the  coronary  liqament  of  the  liver.  The  portion  of  the 
lower  layer  which  forms  the  roof  of  the  grooves  becomes 
the  layer  of  peritoneum  covering  the  posterior  surface  of 
the  upper  layer  (which  represents  the  diaphragm),  while 
the  portion  which  remains  connected  with  the  liver  con- 
stitutes its  peritoneal  investment. 

In  the  mean  time  changes  have  been  taking  place  in  the 
upper  layer.  As  the  rotation  of  the  heart  occurs,  so  that 
its  auricular  portion  comes  to  lie  anterior  to  the  ventricle, 
the  Cuvierian  ducts  are  drawn  away  from  the  septum  and 
penetrate  the  posterior  wall  of  the  pericardium,  the  sepa- 
ration being  assisted  by  the  continued  descent  of  the  at- 
tachment of  the  edge  of  the  septum  to  the  ventral  wall  of 


THE    DIAPHRAGM. 


341 


i 


i 


\ 


the  body  Durins  this  descent,  when  the  upper  layer  of 
the  septum  has  reached  the  level  of  the  fourth  cervical  seg- 
ment a  portion  of  the  myotome  of  that  segment  becomes 
prolonged  into  it  and  the  layer  assumes  the  characteristics 
of  the  diixplmwm,  the  supply  of  whose  musculature  from 
the  fourth  cervical  nerve  through  the  phrenic  is  thus  ex- 
plained. 

The  diaphragm  is  as  yet,  however,  incomplete  dorsally, 
where  the  dorsal  parietal  recesses  are  still  in  continuity 
with  the  trunk-cavity.     With  the  increase  in  thickness  of 
the  septum  transversum,  these  recesses  have  acquired  a 
considerable   length    antero-posteriorly,    and   into   their 
upper  portions  the  outgrowths  from  the  lower  part  of  the 
pharynx  which  form  the  lungs  (see  page  353)  begin  to  pro- 
ject.    The  recesses  thus  become  transformed  into  the 
pleural  cavities,  and  as  the  diaphragm  continues  to  de- 
scend, slipping  down  the  ventral  wall  of  the  body,  and 
drawing  with  it  the  pericardial  cavity,  the  latter  comes 
to  lie  entirely  ventral  to  the  pleural  cavities.     The  free 
borders  of  the  diaphragm,  which  now  form  the  ventral 
boundaries  of  the  openings  by  which  the  pleural  and  peri- 
toneal cavities  communicate,  begin  to  approach  the  dorsal 
wall  of  the  body,  with  which  they  finally  unite,  and  so 
complete  the  separation  of  the  cavities.     The  pleural  cav- 
ities continue  to  enlarge  after  their  separation  and,  ex- 
tending laterally,  pass  between  the  pericardium  and  the 
lateral  walls  of  the  body  until  they  finally  almost  com- 
pletely surround  the  pericardium.    The  intervals  between 
the  two  pleura  form  what  are  termed  the  mediastina  in 
adult  anatomy,  the  posterior  (dorsal)  mediastinum,  in 
which  the  oesophagus  lies,  being  the  remains  of  the  median 
portion  of  tlie  septum  transversum  which  was  attached  to 
the  wall  of  the  gut. 

The  downward  movement  of  the  septum  transversum 


342 


THE    DEVELOPMENT   OF    THE    HUMAN    BODV. 


ie<yUU 


AluAi/ 


i^uU 


extends  through  a  very  considerable  interval,  which  may 
be  appreciated  from  the  diagram  shown  in  Fig.  183. 
From  this  it  may  be  seen  that  in  early  embryos  the  septum 
is  situated  just  in  front  of  the  first  cervical  segr'cnt  and 
that  it  lies  very  obliquely,  its  free  edge  being  decidedly 

posterior  to  its  ventral  attach- 
ment.    When  the  downward 
displacement  occurs,  the  ven- 
tral edge  at  first  moves  more 
rapidly  than  the  dorsal,  and 
soon  comes  to  lie  at  a  much 
lower   level.     The   backward 
movement  continues  through- 
out the  entire  length  of   the 
cei  V  ical  and  thoracic  regions, 
and  when    the    level  of    the 
tenth    thoracic     segment    is 
reached  the  separation  01  the 
p  ,'ural   and   peritoneal  cavi- 
ties  is   completed    and   then 
the  dorsal  edge  begins  to  de- 
scend more  rapidly  than  the 
ventral,    so    that    the     dia- 
phragm   again    becomes    ob- 
lique in   the   same    sense   as 
in  the   beginning,  a  position 
which  it  retains  in  the  adult. 
The  Development  of  the  Peritoneum.— The  peritoneal 
cavity  is  developed  from  the  trunk-cavity  of  early  stages 
and  is  at  first  in  free  communication  on  all  sides  of  the 
volk-stalk  with  the  extra-embryonic  coclom.     As  the  ven- 
tral wall  of  the  body  develops  the  two  cavities  become 
more  and  more  separated,  and  with  the  formation  of  the 
umbilical  cord  the  separation  is  complete.     Along  the 


Fl<;.      18.^.  -  DiAC.RAM     SHOWING 

THE  Position  of  the  Dia- 
phragm IN  Kmbryos  ok  Dif- 
FERE.NT  Ages,—  {Mall.) 


THE    rERITONEUM. 


343 


■s 


mid  dorsal  line  of  the  body  the  archenteron  forms  a  pro- 
jection into  the  cavity  and  later  moves  further  out  from 
the  body-wall  into  the  cavity,  pushing  in  front  of  it  the 
peritoneum,  which  thus  comes  to  surround  the  intestine, 
forming  its  serous  coat,  and  from  it  is  continued  back  to 
the  dorsal  body-wall  forming  the  mesentery. 

It  has  already  been  seen  that  on  the  separation  of  the 
liver  from  tne  septum  transversum,  the  tissue  of  the  latter 
.jives  rise  to  the  peritoneal  covering  of  the  liver  and  of  the 
posterior  surface  of  the  diaphragm,  and  also  to  the  ventral 
mesentery.  When  the  separation  is  taking  place,  the 
rotation  of  the  stomach  already  described  (p.  319)  occurs, 
with  the  result  that  the  portion  of  the  ventral  mesentery 
which  stretches  between  the  lesser  curvature  of  the  stom- 
ach and  the  liver  shares  in  the  rotation  and  comes  to  lie  in 
a  plane  practically  at  right  angles  with  that  of  the  sus- 
pensory ligament,  its  surfaces  looking  dorsally  and  ven- 
trally  and  its  free  edge  being  directed  toward  the  right. 
This  portion  of  the  ventral  mesentery  forms  what  is 
termed  the  lesser  ometUutv.,  and  between  it  and  the  dorsal 
surface  of  the  stomach  as  the  ventral  boundaries  and  the 
dorsal  wall  of  the  abdominal  cavity  dorsally  there  is  a 
cavity,  whose  floor  is  formed  by  the  dorsal  mesentery  of 
tlie  stomach,  the  mesoijastrium,  the  roof  by  the  under 
surface  of  the  left  half  of  the  liver,  while  to  the  right  it 
communicates  with  the  general  peritoneal  cavity  dorsal 
to  the  free  edge  of  the  lesser  omentum.  Tliis  cavity  is 
known  as  the  lesser  sac  of  the  peritoneum,  and  the  opening 
into  it  from  the  general  cavity  or  greater  sac  is  termed 
the  foramen  0}  Winslow.  Later,  the  floor  of  the  lesser 
sac  is  drawn  downward  to  form  a  broad  sheet  of  peri- 
toneum lying  ventral  to  the  coils  of  the  small  intestine 
and  consisting  of  four  layers;  this  represents  the  gnat 
omentum  of  adult  anatomy  (Fig.  187). 


344  T'"-    OKVKLOPMKNI    O       '.UK    'H  M  \N    l.ODV. 

Pclow  the  level  of  the  upper  ..art  oi  the  clu.xlenu.n  the 
ventral  mesenterv  is  vvarains;  only  tlu  d.rsal  tnesenter> 
lurs.     So  ion.  as  the  intestine  is  a  ^tra.^ht  tube      e 
,,„,th  <n  the  intestinal  e<l^e  of  tins  mesenterv  u,    ract  c 
allv  ecmal  to  .hat  of  its  dorsal  attaehed  edge.     The  mt  s 

tne,  however,  inereasin.  in  l-«^\-- V^^TtlXt 
than  the  abdominal  walls,  the  intestmal  ed^e  of  the  mes 
Imerv  soon  becomes  very  much  longer  than  the  attached 

edge,  and  when  the  intestme  grows 
out  into  the  umbilical  cielom  the 
mesenterv  accompanies  it  (I^g- 
184).  As  the  coils  of  the  intes- 
tine develop,  the  intestinal  edge 
of  the  mesenterv  is  thrown  into 
corresponding  folds,  and  on  th 
return  of  the  intestine  to  tlr-  - 
dominal   cavity  the    me  ry  is 

thrown   into  a  somewhat     innel- 
like  form  by  the  twistii.^:  of    the 
intestine  to  form  its  prir.iary  loop 
(Fig.    i«5).      AH  that    portion    of 
the  mesentery  which   is   attached 
to  the  part  of  the  intestin*    vhich 
will    lat.       become    the    jemnuni, 
ileum,    ascending    and    transverse- 
colon,    is    attached    to    tlu    body- 
wall  at  tl,     apex  of  the  lunnel.  at 
a  point  which  lies  to  the  left  of  the  .luodenum 

Up  to  this  stage  or  to  about  the  m.ddle  of  t    e  fou.   n 

month  the  mesentery  has  retained  it>  attachmcrt  to  tl  c 

"dian  line  of  the  dor.al  w.H  of  the  abdomen  tlu  .u.hout 

"entire  length,  but  later  fusion,  of   certam   pcm^;.. 

occur,  wherebv  the  original  condition  t^  greatlv  modtt.  'I 

One  of  the  earliest  of  these  fusions  takes  place  at  the  a   cx 


Fic.  184.      Di.Xf.k AM  SHOW- 
ING THK  .\kk.\n'i:mknt 

OF  THE  MESKNVERY    \NI> 

ViscEKAL  Branches  of 

THE    AHrtOMINAL    AoRTA 

IN   AN    ICmhrvo  of  Six 
Weeks. 

/),   Pancreas;    -S",  stotiiacli ; 
.S"/i,  spleen.      (Toliit.) 


THE  PERITONEUM. 


345 


of  the  fumu'l,  where  tlif  portion  of  thv  mesentery  wliicli 
passes  to  the  tr;  iisvcrse  coh)u  and  at    lies  ovrr  the  <liio(h 
nuni  fuses  with    lie  ventral  su     ice        he  hi    ^r  portion  <  f 


th 


the  intestine  and  also  v  ith  the  peru  »neuti  ■overmK 
dorsal  vail  (.1  tlu  abdoni^-n  be'  h  to  the  Hki  and  to  the 
left  of  the  duodenum.  In  this  \\ay  the  attacliment  of  the 
tntnsvitsi  nn'MKoion  *akes  the  form  of  a  transverse  line 
in^tead  of  a  point,  and  this  portion  of  the  mesentery 


mc 
id 


md 


Fl   .      185         DiA'   KAMS     IlUSTRATIX       THE    DEV     uU^     IEN" 
()^!K^.•T^ :^l    ANl>  TH*-:  Tr  \NSVBRSE   Mk-      <>I 

'■/./,  Cii'cnin;  dd,         W  intestine,  / '.  yijlk-stalk ;  </i. 
gr.  ijrcater  •    ir\aturi-  of  stoi-     ch ;  cj;,  bile  dm 
k,  point  wli<  re  the  1<><  'i)s  of  tli    jnles'  ine  cross 
tuni,  mcs,  li  esi'ii^<T\      w}    vmiiifi      u  a]);)en(liN 


OF   r»H  GkEat 

.iidcniini ; 

a  .  intent  iini; 

i   lion  ;  md,  rec- 

,1:-    ) 


divides  the  alxlominal  cavity  into  two  portions  the  upper 
fanterior)  of  vvhicM  contains  the  liver  and  stomach,  while 
iiR;  lower  contains  tlu  remainder  of  the  digestive  tract 
witli  the  except  on  rf  Jie  duodenuin  ^'v  passing  across 
the  ventral  surface  <  the  duodenum  uiui  fusins,'  with  it, 
thv  transvcr-c  mcsc-  Ion  fop  es  that  porti:'-n  i»f  the  intes- 
tine against  the  dorsal  wall  of  the  abdomen  and  fixes  it  in 
that  position,  and  its  mesentery  thereupon  degenerates, 

29 


H 


346 


THE    DEVELOPMENT    OF   THE    HUMAN    BODY. 


becoming  subserous  areolar  tissue,  the  duodenum  assum- 
ing the  retroperitoneal  position  which  characterizes  it  in 
the  adult. 

The  descending  colon,  which  on  account  of  the  width 
of  its  mesentery  is  at  first  freely  movable,  lies  well  over  to 
the  left  side  of  the  abdominal  cavity,  and  in  consequence 
the  left  layer  of  its  mesentery  lies  in  contact  with  the 
parietal  layer  of  the  peritoneum.  A  fusion  of  these  two 
layers,  beginning  near  the  middle  line  and  thence  extend- 


■-*--Vjr.ya9  . 


^tjKW*'. 


ii 


i' 


i 
I 


i 


Fic.  186. — Diagrams  Iulistratinc  the  Manner  in  which  the  Fixation 
OF  the  Descending  Couon  (C)  takes  Place. 

ing  outward,  takes  place,  the  fused  layers  becoming  con- 
verted into  connective  tissue,  and  this  portion  of  the  colon 
thus  loses  its  mesentery  and  becomes  fixed  to  the  abdomi- 
nal wall.  The  process  by  which  the  fixation  is  accom- 
plished may  be  understood  from  the  diagrams  which 
constitute  Fig.  1 86.  When  the  ascending  colon  is  formed , 
its  mesentery  undergoes  a  similar  fusion,  and  it  also  be- 
comes fixed  to  the  abdominal  wall. 


The  fusion  of  the  mesentery  of  the  ascending  and  descending 
colon  remains  incomplete  in  a  considerable  number  of  cases  (one 


THE    PERITONEUM. 


347 


I 


fourth  to  one-third  of  all  cases  examined),  and  in  these  the 
colons  are  not  perfectly  fixed  to  the  abdominal  wall.  It  may 
also  be  pointed  out  that  the  caecum  and  appendix,  being  pri- 
marily a  lateral  outpouching  of  the  intestine,  do  not  possess  any 
true  mesenterv,  but  are  completely  enclosed  by  peritoneum. 
Usually  a  falciform  fold  of  peritoneum  may  be  found  extending 
along  one  surface  of  the  appendix  to  become  continuous  with 
the  left  laver  of  the  mesentery  of  the  ileum.  This,  however,  is 
not  a  true  mesenter>  ,  md  is  better  spoken  of  as  a  mesenteriole. 

One  other  fusioti  is  still  necessary  before  the  adult  condi- 
tion of  the  mesentery  is  acquired.  The  great  omentum 
consists  of  two  folds  of  peritoneum  which  start  from  the 
greater  curvature  of  the  stomach  and  pass  downward  to 
be  reflected  up  again  to  the  dorsal  wall  of  the  abdomen, 
which  they  reach  just  anterior  to  (above)  the  line  of 
attachment  of  the  transverse  mesocolon  (Fig.  187,  A). 
At  first  the  attachment  of  the  omentum  is  vertical,  since 
it  represents  the  mesogastrium,  but  later,  by  fusion  with 
the  parietal  peritoneum  it  assumes  a  transverse  direction, 
while  at  the  same  time  the  pancreas,  which  originally  lay 
between  the  two  folds  of  the  mesogastrium,  is  carried 
dorsally  and  comes  to  have  a  retroperitoneal  position  in 
the  line  of  attachment  of  the  omentum.  By  this  change 
the  lower  layer  of  the  omentum  is  brought  in  contact  with 
the  upper  layer  of  the  transverse  mesocolon  and  a  fusion 
and  degeneration  of  the  two  results  (Fig.  187,  B),  a  condi- 
tion which  brings  it  about  that  the  omentum  seems  to  be 
attached  to  the  transverse  colon  and  that  the  pancreas 
seems  to  lie  in  the  line  of  attachment  of  the  transverse 
mesocolon.  This  mesentery,  as  it  occurs  in  the  adult, 
really  consists  partly  of  a  portion  of  the  original  trans- 
verse mesocolon  and  partly  of  a  layer  of  the  great  omen- 
tum. 

By  these  various  changes  the  line  of  attachment  of  the 
mesentery  to  the  dorsal  wail  of  the  body  has  become  some- 


348 


THE    DEVELOPMENT   OF    THE    HUMAN    BODY. 


what  complicated  and  has  departed  to  a  very  considerable 
extent  from  its  original  simple  vertical  arrangement.  If 
all  the  viscera  be  removed  from  the  body  of  an  adult  and 
the  mesentery  be  cut  close  to  the  line  of  its  attachment, 
the  course  of  the  latter  will  be  seen  to  be  as  follows :  De- 
scending from  the  under  surface  of  the  diaphragm  are  the 


Fio.  187      Di.viKAMS  sHowiNi;  THE  Deveuoi'ment  of  the  Great  Ome.n- 

TiM  AND  ITS  Fusion  with  the  Tsansverse  Mesocolon. 

H,  Bladder;  c,  transverse  colon;  J,  duodenum;  Li.  liver;  />,  pancreas;  A", 

rectum;  .S",  stttmach;  I',  uterus. —  (.'l//<r  AlUn  Tliom.uin.) 


I 


lines  of  attacliment  of  the  suspensory  ligament,  which  on 
reaching  the  liver  spread  out  to  become  t!ie  coronary  and 
lateral  ligaments  of  tliat  organ.  At  about  the  mid-dorsal 
line  these  lines  become  continuous  with  those  of  the 
mesogastrium  which  curve  downward  toward  tlie  left  and 
are  continued  into  the  transverse  lines  of  the  transverse 


THE   SPLEEN. 


349 


f 


mesocolon.  Between  these  last,  in  a  sliglit  prolongation, 
there  may  be  seen  to  the  right  the  cut  end  of  the  first  por- 
tion of  the  duodenum  as  it  passes  back  to  the  dorsal  wall 
of  the  abdomen,  and  at  about  the  mid-dorsal  line  the  cut 
ends  of  its  last  part  become  visible  as  it  passes  ventrally 
again  to  become  the  jejunum.  From  the  transverse 
mesocolon  three  lines  of  attachment  pass  downward ;  the 
two  lateral  broad  ones  represent  the  lines  of  fixation  of 
the  ascending  and  descending  colons,  while  the  narrower 
median  one,  which  curves  to  the  right,  represents  the  at- 
tachment of  the  mesentery  of  the  small  intestine  other 
than  the  duodenum.  Finally,  from  the  lower  end  of  the 
fixation  line  of  the  descending  colon  the  ^lesentery  of  the 
sigmoid  is  continued  downward.  s" 

The  Development  of  the  Spleen.— The  spleen  has  gen- 
erally  been  regarded  as  a  development  of  the  mesenchyme 
situated  between  the  two  layers  of  the  mesogastrium.  To 
this  view,  however,  recent  observers  have  taken  exception, 
holding  that  the  ultimate  origin  of  the  organ  is  in  part  or 
entirely  from  the  coelomic  epithelium  of  the  left  layer  of 
the  mesogastrium.  The  first  indication  of  the  spleen  has 
been  observed  in  embryos  of  the  fifth  week  as  a  slight 
elevation  on  the  left  (dorsal)  surface  of  the  mesogastrium, 
due  to  a  local  thickening  and  vascularization  of  the  mesen- 
chyme accompanied  by  a  thickening  of  the  coelomic  epi- 
thelium which  covers  the  elevation.  The  mesenchyme 
thickening  presents  no  differences  from  the  neighboring 
mesenchyme,  but  the  epitheHum  is  not  distinctly  Sepa- 
rated from  it  over  its  entire  surface,  as  it  is  elsewhere  in 
the  mesentery.  In  later  stages,  which  have  been  ob- 
served in  detail  in  pig  and  other  amniote  embryos,  cells 
separate  from  th.e  deeper  layers  of  the  epithelium  (Fig. 
1 88)  and  pass  into  the  mesenchyme  thickening,  whose 
tissue  soon  assumes  a  different  appearance  from  the  sur- 


I 


350 


THE    DEVELOPMENT    OF    THE    HUMAN    BODY 


rounding  mesenchyme  by  its  cells  being  much  crowded. 
This  migration  soon  ceases,  however,  and  in  embryos  of 
forty-two  days  the  coelomic  epithelium  covering  the  thick- 
ening is  reduced  to  a  simple  layer  of  cells. 

The  later  stages  of  development  consist  of  an  enlarge- 
ment of  the  thickening  and  its  gradual  constriction  from 
the  surface  of  the  mesogastrium,  until  it  is  finally  united 
to  it  only  by  a  narrow  band  through  which  the  large 
splenic  vessels  gain  access  to  the  organ.  The  cells  differ- 
entiate themselves  into  trabeculae  and  pulp  cords,  special 
collections  of  cells  around  the  branches  of  the  splenic 
artery  forming  the  Malpighian  corpuscles. 


Fi. 


188.     Section  THRort.n  the  Left  Layer  of  the  Mesooastrium 
OF  A  LniLK  Kmbrvo  ok  Ninety-thkee  Hours,  sHowiNr.  the  Oricin 

OF  THE  Sri.EEN. 

efy,  Cftltiiiiic  epithelium;  ms,  inescncliyine.     ( lonkoff.) 


It  has  already  been  fK)inted  out  (p  ^44)  that  during  embry- 
onic life  the  spleen  is  an  important  h:pmatopoietic  organ,  both 
red  and  white  corpuscles  undergoing  active  formation  within 
its  substance.  The  Malpighian  corpus*  les  are  collections  nf 
lymphocytes  in  which  multiplication  takes  place,  and  while 
nothing  is  as  yet  known  as  to  the  fate  of  the  cells  which  are 
contributed  to  the  spleen  from  the  roelomic  epithelium,  since 
they  quickly  come  to  resemble  the  rn'  ^^cnchyme  cells  with  which 
thev  dJF*-  associated,  vet  thf  jjTowiiij(  number  of  observations 
indicatmg  an  epitheHal  origin  lor  lymphocy!'^  suggests  the 
pr)ssibility  that  the  cells  iE  qurstion  may  lie  responsible  for  the 
first  leukocytes  of  the  sjA-m. 


LITERATURE. 


351 


I 

s  ■ 

I 


LITERATURE. 

A.  BracheT:  "  Die  Kntwickelung  der  grossen  Kiirperhohlen  und  ihre  Tren- 

nung  von  Einander,"  Ergebnisse  der  Anat.  und  Entu'ickelungsgesch ., 

VII,  1898. 
\\'.  His:  "Mittheilungen  zur  Embryologie  der  Saugethiere  und  des  Mens- 

chen,"  Archiv  fur  Aval,  und  Physiol.,  Anat.  Abth.,  1881. 
F.  P.  Maul:  "Development  of  the  Human  Coelom,"  Journal  0}  Morphol., 

XII,  1897. 

E.  Ravn:  "Ueber  die   Bildung  der  Scheidewand  zwischen   Brust-  und 

Bauchhohle  inSaugetliierembryonen,"  Archiv  \ur  Anat.  urd  Physiol., 

Atmt.  Abth.,  1889. 
A.  Swaen:  "  Recherches  sur  le  developpement  du  foie,  du  tube  digestif,  de 

rarri^re-cavjte  du  peritoine  et  du  m^senttre,"  Journ.  dc  I  'Anat.  ct  dc 

la  Physiol.,  xxxii,  1896;  xxxiii,  1897 
C.  TouuT:  "  Bau  und  Wachsthumsveranderungen  der  Gekriise  des  mensch- 

lichen  Darmkanals,"    Denkschr.  der  kais.    Akad.    Wissensch.   Wiin, 

Math.-Xaturwiss.  Classc,  xli,  1879. 
C.  ToldT:  "  Die  Darnigekriise  und  Netze  im  gesetzmassigen  und  gesetzwi- 

drigen  Zustand,"  Denk.'tchr.  der  kais.  Akad.  Wissensch.  Wien,  Math.- 

Naturwiss.  Classe,  LVI,  1889. 
W.  Tonkopf:  "Die  Entwickelung  der  Milz  bei  den  Aninioten,"  Archiv  jiir 

mikrosk.  .\nat.,  LVI,  1900. 

F.  Treves:  "  Lectures  on  the  Anatomy  of  the  Intestinal  Canal  and  Perito- 

neum," Hritish  Medical  Journal,  I,  1885 


1= 


CHAPTER    XII. 

THE    DEVELOPMENT    OF    THE    ORGANS    OF 
RESPIRATION. 

The  Development  of  the  Lungs.— The  first  indication 
of  the  lungs  and  trachea  is  found  in  embryos  of  about  3.2 
mm.  in  the  form  of  a  groove  on  the  ventral  surface  of  the 
oesophagus,  at  first  extending  almost  the  entire  length  of 
that  portion  of  the  digestive  tract.     As  the  oesophagus 

lengthens  the  lung  groove 
remains  connected  with  its 
upper  portion  (Fig.  168, 
A),  and  furrows  which  ap- 
pear along  the  line  of 
junction  of  the  groove 
and  the  oesophagus  gradu- 
ally deepen  and  separate 
the  two  structures  (Fig. 
168,  B).  The  separation 
takes  place  earliest  at  the 
lower  end  of  the  groove 
and  thence  extends  up- 
ward, so  that  the  groove 
is  transformed  into  a  cyl- 
indrical pouch  lying  ven- 
trad  of  the  (esophagus 
and  dorsad  of  the  heart  and  opening  with  the  cesophagus 
into  the  terminal  portion  of  the  pharynx. 

Soon  after  the    separation  of    the    groove   from    the 
oesophagus  its  lower  end  becomes  enlarged  and  bilobed, 

352 


Tk..       1S<;.        TOKTION      or     A     SlXTION 
TMROIf.M      .W       HmBRSO       Ol-      TIIK 

FoiKTH  Week. 
,1,  .\orta;    DC,   ductus   Cuvieri;    L, 
lunjj;  (',  (I'sopliagus;  A'/',   parietal 
recess;    I'l'w,    omiilialo-inescnteric 
vein.     (Toldt.) 


THE    LUNGS. 


353 


and  since  this  lower  end  lies,  with  the  cesophagus,  in  the 
median  attached  portion  of  the  dorsal  edge  of  the  septum 
transversum,  the  lobes,  as  they  enlarge,  project  into  the 
dorsal  parietal  recesses  (Fig.  189),  and  so  become  enclosed 
within  the  peritoneal  lining  of  the  recesses  which  later 
become  the  pleural  cavities. 

The  lobes,  which  represent  the  lungs,  do  not  long  remain 
simple,  but  bud-like  processes  arise  from  their  cavities, 
three  appearing  in  the  right  lobe  and  two  in  the  left  (Fig. 


J 


E?* 


A 


Imc.    190.— Reconstruction  ok  the  Lunc.  Outgrowths  of  Embryos 

OF  (/I)    10,   (B)   8.5,   AND  (C)    10.5   MM. 

Al>,  Pulmonary  artery ;  £/>,  apical  bronchus;   Vp,  pulmonary  vein ;  /-//, 
primary  bronchi.  -{His.) 

190,  A),  and  as  these  increase  in  size  and  give  rise  to  addi- 
tional outgrowths,  the  structure  of  the  lobes  rapidly  be- 
comes complicated  (Fig.  190,  B  and  C).  In  the  formation 
of  new  outgrowths  the  terminal  enlarged  part  of  each  pro- 
cess divides  as  if  to  give  rise  to  two  equal  bronchi,  but 
later  as  the  new  bronchi  elongate,  one  grows  more  rapidly 
than  the  other  and  places  itself  so  as  to  be  in  the  line  of  the 
stem  from  which  it  arose,  its  fellow  seeming  to  be  a  lateral 
branch  from  it.  As  a  result  of  this  method  of  growth  a 
30 


MM 


354 


THE    UEVEI.orMKNT    OF    THE    HUMAN    IIODY. 


main  bronchus  traversing  the  entire  length  of  the  lung  is 
formed,  and  into  it  there  open  numerous  lateral  branches, 
which  may  be  termed  secondary  bronchi,  arranged  in  a 
more  or  less  definite  and  similar  manner  in  the  two  lungs. 
The  main  stem  of  the  pulmonary  artery  traverses  the  lung 
lying  to  the  outer  side  of  the  main  bronchus,  and  since  cer- 
tain of  the  secondary  bronchi  arise  ventral  and  others  dor- 
sal to  the  line  of  the  artery  it  is  possible  to  recognize  series 
of  ventral  and  dorsal  bronchi.  These  alternate  more  or 
less  regularly  with  one  another,  the  dorsal  bronchi  stand- 
ing higher  than  the  ventral  and  in  the  human  lung  there 
are  usually  four  ventral  bronchi,  while  the  number  of  the 
dorsal  ones  is  frequently  reduced  to  three  by  the  failure  of 
the  one  corresponding  to  the  third  ventral  to  develop. 

The  first  dorsal  bronchus  of  the  left  side  differs  from 
that  of  the  right  side  in  that  it  arises  from  the  first  ventral 
bronchus  instead  of  from  tlie  main  stem,  a  condition  with 
which  is  associated  the  fusion  of  the  upper  and  middle 
lobes  of  the  left  lung  to  a  single  lobe. 

The  secondary  branches  elongate  and  give  rise  to  lateral 
branches  just  as  do  the  main  bronchi,  and  of  these  tertiary 
bronchi  one,  which  arises  from  the  second  ventral  bron- 
chus or  from  tlie  main  bronchus  in  its  neighborhood,  is  of 
especial  importance,  since,  especially  in  the  right  lung,  in 
which  it  is  usually  better  developed  than  in  the  left,  it 
frequently  forms  the  main  stem  for  a  fourth  lobe,  which, 
from  its  position,  is  termed  the  injracardial  lobe. 

At  first  the  amount  of  mesenchyme  which  separates 
the  various  branches  is  comparatively  great,  but  as  the 
branchings  continue,  the  growth  of  the  mesenchyme  fails 
to  keep  pace  witli  it,  so  that  in  later  stages  the  terminal 
enlargements  are  separated  from  one  another  by  only  very 
thin  partitions  of  mesenchyme  in  which  the  pulmonary 
vessels  form  a  dense  network.     The  final  branchings  of 


M*i 


TIIK    I.AKYNX. 


355 


i 


each  ultimate  bronchus  or  bronchiole  results  in  the  forma- 
tion at  its  extremity  of  from  three  to  five  enlargements, 
the  atria  (Fig.  191,  .4  ),  from  which  arise  a  number  of  air- 
sacs  or  alveoli  (s)  whose  walls  are  pouched  out  into  slight 
diverticula,  the  air-cells.  Such  a  combination  of  atria, 
air-sacs,  and  air-cells  constitutes 
a  lobule,  and  each  lung  is  com- 
posed of  a  large  number  of  such 
units. 

The  greater  part  of  the  origi- 
nal pulmonary  groove  becomes 
converted  into  the  trachea,  and 
in  the  mesenchyme  surrounding 
it  the  incomplete  cartilaginous 
rings  develop  at  about  the 
eighth  or  ninth  week.  The  cells 
of  the  epithelial  lining  of  the 
trachea  and  bronchi  remain  col- 
umnar or  cubical  in  form  and 
become  ciliated  at  about  the 
fourth  month,  but  those  of  the 
epithelium  of  the  air-sacs  be- 
come greatly  flattened  and  con- 
stitute an  exceedingly  thin  layer 
of  pavement  epithelium. 

The  Development  of  the  Larynx.-  The  opening  of  the 
upper  end  of  the  pulmonary  groove  into  the  pharynx  is 
situated  at  first  just  behind  the  fourth  branchial  furrow 
and  is  surrounded  anteriorly  and  laterally  by  the  n- 
sl'.aped  ridge  already  described  (p.  3 1 1 )  as  the  furcula,  this 
.separating  it  from  the  posterior  portion  of  the  tongue  (Fig. 
164).  The  anterior  portion  of  this  ridge,  which  is  appar- 
ently derived  from  the  ventral  portions  of  the  third  bran- 
cliial  arcJi,  gradually  increases  in  height  and  forms  the 


Fic.  191.  -  Di,\(;k.\m  oh  the 
Final  Branches  of  the 
Mammalian  Hronchi. 

.1,  .\triuni;   li,  bronchus;  .S, 
air-sac. — (Miller.) 


356 


THE    OEVEIOPMENT   OF   THE    HUMAN    BODV. 


V 


epiglottis,  while  the  lateral  portions,  which  pass  posteriorly 
into  the  margins  of  the  pulmonary  groove,  form  the  aryte- 
noid ridges.  When  the  pulmonary  groove  separates  from 
theoesophagus,theopeningof  the  trachea  into  the  pharynx 
is  somewhat  slit-like  and  is  bounded  laterally  by  the  aryte- 
noid ridges,  whose  margins  present  two  elevations  which 
may  be  termed  the  cornicular  and  cuneiform  tubercles 
(Fig.  192,  CO  and  cu,  and  Fig.  161 ).  The  opening  is,  how- 
ever, for  a  time,  almost 
obliterated  by  a  thicken- 
ing of  the  epithelium 
covering  the  ridges,  and 
it  is  not  until  the  tenth 
or  eleventh  week  of  de- 
velopment that  it  is  re- 
established. Later  than 
this,  at  the  middle  of  the 
fourth  month,  a  linear 
depression  makes  its  ap- 
pearance on  the  mesial 
wall  of  each  arytenoid 
ridge,  forming  the  begin- 
ning of  the  ventricle,  and 
nlthough  at  first  the  de- 
pression lies  horizontally 
its  lateral  edge  later 
bends  ai  »eriorly,  so  that  its  surfaces  look  outwards  and 
inwards.  The  lips  which  bound  the  opening  of  the  ven 
tricle  into  the  laryngeal  cavity  give  rise  to  the  vocal  cords. 
The  cartilages  of  the  larynx  can  be  distinguished  durin^^ 
the  seventh  week  as  condensations  of  mesenchyme  which 
are  but  indistinctly  separated  from  one  another.  The 
thyreoid  cartilage  is  represented  at  this  stage  by  two  lateral 
plates  of  mesenchyme,  separated  from  one  another  both 


Fig.  192. — Ricco.nstruction-  of  the 
Opening  into  the  I,\ryn.\  in  an 
Kmbrvo  of  T\\  icn'Tv  kii.ht  Days, 
Seen  from  Behi  :>  ANii  .\bove,  the 
DoRs.M<  Waul  of  ihe  I'hxrvnx  be- 
ing Cl'T  AWAV. 

CO,  Ci)rnicular,  and  cu,  cuneiform 
tubercle;  Ef^.  epigl.  ;tis;  T,  unpaired 
portion  of  tl     tongue. — {Kalliir^.) 


i       i 


TMK    I.AKYNX. 


357 


veiitrally  and  dorsally,  and  t-ach  of  ihtsc  plates  undergtjcs 
chondrificalion  from  two  separate  centers  (Fig.  193)- 
These,  as  they  increase  in  size,  unite  together  and  send 
prolongations  vent'  dly  which  meet  in  th^-  mid-ventral 
line  with  the  corres,>onding  prolongations  of  the  plates  of 
tilt'  opposite  side,  so  as  to  enclose  an  area  of  mesenchyme 
into  which  the  chondrification  only  extends  at  a  later 
period,  and  occasionally  fails  to  so  extend,  producing  what 
is  termed  a  foramen  thyreoideum. 

The  mesenchymal  condensations  which  represent  the 
cricoid  and  arytenoid  cartilages  are  continuous,  but  each 


Fin.  19.^. — Reconstruction  ok  the  Mbsiv-chyme  Conhensations  which 

Represent  the  Hvoio  and  Thyreoid  Cartilages  in  an  li.MBRYO 

of  Forty  Days. 
The    darkly   shaded  areas    represent    center'^  of    chondrification.     c.ma, 

Greater  cornu  of   hyoid;  c.mi,  lesser  cornu;    T!i,  thyreoid  cartilage. 

—{Kallius.) 

arytenoid  has  a  distinct  center  of  chondrification,  while 
the  cartilage  of  the  cricoid  appears  as  a  single  ring  which 
is  at  first  open  dorsally  and  only  later  becomes  complete. 
The  epiglottis  cartilage  resembles  the  thyreoid  in  being 
formed  by  the  fusipn  of  two  originally  distinct  cartilages, 
from  each  of  which  a  portion  separates  to  form  the  cunei- 
form cartilages  {coHilagrs  of  \\  risberg),  while  the  cornic- 
ulae  Ipt-yngis  {cartilages  of  Saniorini)  are  formed  by  the 
separation  of  a  small  portion  of  cartilage  from  each  aryte- 
noid. 


1 


358 


TIIH    OEVEI-OI'MENT   OK    THK    IIIMAN    BODV. 


•    j. 


The  foniKitJon  of  the  thyreoid  eartihige  by  the  fusion  of 
two  pairs  of  lateral  elements  finds  an  explanation  from 
the  study  of  the  comparative  anatomy  of  the  larynx.  In 
the  lowest  group  of  tlie  mammalia,  the  Monotremata,  the 
four  cartilages  do  not  fuse  together  and  are  very  evidently 
serially  homologous  with  the  cartilages  which  form  the 
cornua  of  the  hyoid.  In  other  words,  the  thyreoid  results- 
from  the  fusion  of  the  fourth  and  fifth  branchial  cartilages. 
The  cricoid,  in  its  development,  presents  such  striking 
similarities  to  the  cartilaginous  rings  of  ihe  trachea  that  it 
is  probably  to  be  regarded  as  the  uppermost  cartilage  of 
that  series,  but  the  paired  arytenoids  and  the  epiglottis 
are  possibly  representatives  of  the  sixth  and  seventh 
pairs  of  branchial  cartilages,  structures  which  occur  with 
great  constancy  in  the  lower  vertebrates.  The  epiglottis 
possibly  represents  the  sixth  pair  of  cartilages  and  the 
arytenoids  the  seventh  (Gegenbaurj. 

These  two  last  arches  have  undergone  almost  complete 
reduction  in  the  mammalia,  the  cartilages  being  their  only 
representatives,  but,  in  addition  to  the  cartilages,  the 
fourth  and  fifth  arches  have  also  preserved  a  portion  of 
their  musculature,  part  of  which  becomes  transformed 
into  the  muscles  of  the  larynx.  Since  the  nerve  which 
corresponds  to  these  arches  is  the  vagus,  the  supply  of  the 
larynx  is  derived  from  that  nerve,  the  superior  laryngeal 
nerve  probably  corresponding  to  the  fourth  arch,  while 
the  inferior  (recurrent)  answers  to  the  fifth. 


t    ij 


•i  I    j^ 


The  course  of  the  recurrent  laryngeal  mi  vc  finds  its  explana- 
tion in  the  relation  of  the  nerve  to  the  fourth  braiwhial  artery. 
When  the  heart  occupies  its  primary  ]xjsition  ventral  to  the  floor 
of  the  pharynx,  the  inferior  laryngeal  nerv"  passes  transversely 
inward  to  the  lar>nx  beneath  the  fourth  branchial  artery.  As 
the  heart  recedes  the  nerve  is  caught  by  the  vessel  and  is  carried 
back  with  it,  the  portion  of  the  vagus  between  it  and  the  supe- 
rior laryngeal  nerve  elongating  until  the  origins  of  the  t  vo 


->«ti 


I.lTKRATn   I'.. 


^59 


laryngeal  norvo  an-  si  paratid  hy  tin  entire  KiikI  i  «'i  Ui.  imk. 
Hence  it  is  that  the  right  reenrreiii  nerve  heiid-  upward  1..  liiiuJ 
the  ri>;li(  sulx^lavian  artery,  while  tfie  left  eiirv.  heneai  tin 
arch  of  the  aorta  (see  Imk    i.V))- 


LITERATURE. 
K.  C.OPPFKT    "  t  flH.T  dii'    'iTkiinft  (ii  r  Wristx-iusclu  u  Kn.irii.  Is    \l<trf^h>>l. 

Jiihr   u.h    XXI,  I8«)4. 
W.    His      ■/.        I«ildiinj{SKesehicliU'  >liT  I.iinKtn   In-iin   tin  nscliliclien   Iviu- 

liryd,  '  ArckivJHr  Aiuit.  uml  /Vuu"/.,    '"'''    Ahlh.,  IKK7 
I-:.    Kalui  "KeitriiKf  ziir  Km  «ickelun^;^k   -cliiilitc    li      Kclilkopfes,'" 

\»,il.  Ii^    '•'.  IX.   1H')7 
H    K\i.uii's;       Ihi-     F':nt\vickeluii.   di-s  mens*  1  Hi  lien   k-hlkopfes,"  ff- 

Imu.ll.  (in  Anal,  (irull.rh.,  rcii     l«'>H. 
A.    NaraTII:   "  Dir   Hmncliiall'aiini  <ter  vSauKelliierf  iiiul  dis  Mcnsclien," 

lUhlioihna  \trilHii.  Al.th.  A.  Uift  ^,  l')l)l. 


t  ii 


lit 


i  I. 


.    it  I 

)     Ii    I 


i    i 


•:\l 


CHAPTER  XIII. 

THE  DEVELOPMENT  OF  THE  URINOGENITAL 
SYSTEM  AND  THE  SUPRARENAL  BODIES. 

The  excretory  and  reproductive  systems  of  organs  are 
so  closely  related  in  their  development  that  they  must  be 
considered  together.  They  both  owe  their  origin  to  the 
mesoderm  which  constitutes  the  intermediate  '^ell-mass, 
this,  at  an  early  period  of  development,  becoming  thick- 
ened so  as  to  form  a  ridge  projecting  into  the  dorsal  por- 


/ir 


/ 


\ 


.11 


'\ 


Fig.  194 — Transverse  Section  through  the  Abdominal  Region  of 
A  Rabbit  Kmbryo  of  12  mm. 

o,  Aorta;  g\,  glomerulus;  gr,  genital  ridge;  m,  mesentery;  wc,  notochord; 
/,  tubule  of  mesonephros ;  W,  Wolffian  duct;  tut.  Wolffian  ridge. — 
{\Uhalk(rvkz^ 

tion  of  the  coelom  and  forming  what  is  known  as  the  Wolf- 
fian ridge  (Fig.  194,  w>).  The  greater  portion  of  the  sub- 
stance of  this  ridge  is  concerned  in  tlie  development  of  the 
primary  and  secondary  excretory  organs,  but  on  its  mesial 
surface  a  second  ridge  appears  which  is  destined  lO  give 
rise  to  the  ovary  or  testis,  and  Ijfijice  is  termed  the  genital 

ridge  ig). 

360 


THE   PRONEPHROS. 


361 


The  development  of  the  excretory  organs  is  remarkable 
in  that  three  sets  of  organs  appear  in  succession.  The 
first  of  these,  the  pronephros,  exists  in  a  very  rudimentary 
condition  in  the  human  embryo,  although  its  duct,  the 
pronephric  or  Wolifian  duct,  undergoes  complete  develop- 
ment and  plays  an  important  part  in  the  development  of 
the  succeeding  organs  of  excretion  and  also  in  that  of  the 
reproductive  organs.  The  second  set,  the  mesonephros 
or  Wolifian  body,  reaches  a  considerable  development  dur- 
ing embryonic  life,  but  later,  on  the  development  of  the 
final  set,  the  definitive  kidney  or  metanephros,  undergoes 
degeneration,  portions  only  persisting  as  rudimentary 
structures  associated  for  the  most  part  with  the  reproduc- 
tive organs. 

The  Development  of  the  Pronephros  and  the  Proneph- 
ric Duct.— Thf  first  portions  of  the  excretory  system  to 
make  their  appearance  are  the  pronephric  or  WolfSan 
ducts,  and  these  develop  as  thickenings  of  the  lateral  parts 
of  the  intermediate  cell-masses.  At  first  the  thickenings 
form  solid  cords  of  cells  (Fig.  195,  wd),  but  later  a  lumen 
appears  in  the  center  of  each  cord,  which  thus  becomes 
converted  into  a  canal.  In  early  stages  the  cords,  toward 
their  posterior  ends,  may  undergo  a  secondary  fusion  with 
the  immediately  overlying  ectoderm  (Martin)  and  may 
thereby  present  the  appearance  of  having  arisen  from  that 
layer,  but  when  fully  developed  the  ducts  lie  in  the  sub- 
stance of  the  Wolffian  ridges  (Fig.  194,  wd),  their  anterior 
ends  being  situated  well  forward  in  the  region  occupied  by 
the  heart,  whence  they  extend  backward  to  open  on  the 
ventral  part  of  the  Irt.ral  walls  of  the  cloaca  (Fig.  156). 

The  pronephros  appears  in  embryos  of  about  3  mm.  as 
two  tubular  invaginations  of  the  ccelomic  epitheliu.n  into 
the  substance  of  each  WolflRan  ridge,  in  the  region  in  which 
the  anterior  end  of  the  Wo.„'an  duct  is  found  (Janhosik). 


HBHHi 


362 


THE    DFAELOPMENT    OF    THE    HUMAN    HODV. 


I' 


The  tubules  do  not  proceed  to  complete  development, 
making  no  connection  with  the  duct,  and  indeed  the  ante- 
rior one  hardly  deserves  to  be  termed  a  tubule,  since  it  is 
a  solid  cord  of  cells,  continuous  at  one  extremity  with  the 
coelomic  epithelium.  The  posterior  one  is,  however,  a 
hollow  tubule  ending  blindly  at  one  extremity,  while  at 
the  other  it  communicates  with  the  coelomic  cavity,  the 
opening  being  termed  a  nephrostome.  Opposite  these  ru- 
dimentary tubules  there  arises  from  the  root  of  the  mesen- 
tery a  process  which  projects  freely  into  the  coelom  toward 
the  nephrostomes.     This  probably  represents  a  rudimen- 


!  )• 


/u: 


Fig.    195.— Transverse  Section  throic;!!  Chick    ICmbryo  ok    aboit 

Thirty-six  Hoirs. 
en,   Rndoderm;    im,    intermediate    cell    mass;    mx,    mesodennic    somite- 

nc,  i.otochord ;  JO,  somatic,  and  xp,  splanchnic  mesoderm ;  li,/,  Wolflian' 

duct.—dVa'dcytr.) 


■I  I 


ill 


tary  free  glomerulus,  into  which  branches  from  the  aorta 
may  project. 

Nothing  is  known  as  to  the  further  development  of  these 
proncphric  tubules  and  glomeruli,  but  it  seems  probable 
that  they  are  merely  transitory  structures  which  disap- 
pear completely  at  an  early  stage  of  development  (see 
P-390- 

A  similar  but  more  perfectly  devektped  pronepL'-ns  has  .  ..-n 
described  in  other  mammals,  such  as  the  rahhit  and  ra'  and  is 
of  constant  occurrence  in  all  the  iower  vtrtebiates.  In  thest 
the  proncphric  tubules,  which  may  be  six  <'in  the  lamprev  )  or 
more  in  number  on  each  side,  are  primarilv  arranged  segment 


^1 


1 


THK    MKSONKPIIKOS. 


363 


ally,  and  open  hv  one  extremity  into  the  anterior  portion  of  the 
Wolffian  duct  and  bv  the  other  into  the  co-lomic  cavity,  and, 
furthermore,  each  tubule  has  corresponding  to  it  a  glomerulus 
which  lies  freelv  in  the  ccclomic  cavity  in  the  vicinity  of  the 
nephr<«tome.  Bv  these  free  glomeruli  and  by  the  possession 
of  nephrostomes  the  tubules  of  the  i)ronephros  are  distinguished 
from  those  of  the  mesonephros  in  the  higher  vertebrates,  and 
since  both  these  peculiarities  are  represented  in  the  two  pairs  of 
tubules  described  above  as  occurring  in  the  ;^  mm.  human  em- 
bryo, there  seems  to  be  little  room  for  doubt  but  that  they  arc 
representatives  of  a  rudimentary  pronephros. 

It  has  been  very  generally  supposed  that  the  tubules  of  the 
mesonephros,  which  develop  in  the  segments  succeeding  those 
which  contain  the  pronephnis,  were  serially  homologous  with 
the  pronephric  tubules.  Doubts  have  recently  been  aroused 
against  this  theory  (Ruckert,  Wheeler).  Important  structural 
differences  exist  in  the  two  sets  of  tubules,  and  since  even  in  the 
lowest  vertebrates  the  pronephros  seems  to  be  a  rudimentary 
structure,  it  has  been  held  not  improbable  that  in  the  ancestors 
of  the  vertebrates  it  was  a  much  more  perfectly  developed  organ, 
extending  back  into  the  region  occupied  by  the  mesonephros  in 
existing  vertebrates.  As  the  mesonephros  developed  the  pro- 
nephros underwent  degeneration,  portions  of  its  tubules  per 
sisting,  however,  and  uniting  to  form  a  continuous  canal,  the 
pronephric  duct,  a  structure  for  which,  otherwise,  it  is  difficult  to 
find  a  satisfactor\-  explanation.  The  fact  that  in  lower  forms 
the  duct  seems  to  develop  as  a  number  of  separate  parts  which 
later  become  continuous  '"^  ..uds  in  favor  of  this  hypothesis,  but 
in  opposition  to  it  h  the  observation  that  the  lower  portion  of 
the  duct  in  several  species  of  mammals  arises  from  the  ectoderm 
(von  Spec.  Flemming).  It  seems,  however,  to  be  established 
that  in  the  majority  of  the  lower  vertebrates  it  is  of  purely 
mesodermal  origin,  and  its  connection  with  the  ectoderm  in  the 
riianmialia  is  therefore  very  probably  due  to  a  secondary  fusion 
(Martin). 


4fe 


The  Development  of  the  Mesonephros. — The  pronephric 
duct  does  not  disappear  with  the  det^eneration  of  the  pro- 
nephric tubules,  but  persists  to  serve  as  the  duct  for  the 
mesonephros  and  to  play  an  important  part  in  the  devel- 
opment of  the  metanephros  also.  In  the  regions  of  the 
Wolffian  ridge  which  lie  posterior  to  the  pronephros  there 


364 


TIIK    DKVKI.Ot'MKNT    Ol-     THE    HUMAN    BODY. 


i 


I       -.n 


i   l'*" 


WJ 


appear  in  embryos  of  between  3  and  4  mm.  a  number  of 
coiled  tubules  whose  origin  has  not  yet  been  sufficientlv 
elucidated  in  human  embryos.  In  lower  mammals  tliey 
arise  by  some  of  the  cells  of  the  Wolffian  ridge  aggregating 
together  to  form  solid  cords,  which  are  entirely  uncon- 
nected with  the  coelomic  epitlielium  and  at  first  also  with 
the  Wolffian  (pronephric)  duct.  These  cords  acquire  a 
lumen  and  at  one  end  connect  with  the  duct,  while  near 
the  other  end  a  condensation  of  the  mesoderm  of  th«-  ridge 

occurs  to  form  a  glomer- 
ulus into  which  a  vessel 
extends  from  the  i>eigh- 
boring  aorta.  The  tu- 
bules rapidly  increase  in 
k  Tigth  and  become 
coiled,  and  the  glomeruli 
project  into  their  cavi- 
ties, pushing  m  front  of 
them  the  wall  of  the  tu- 
bule so  that  the  whole 
structure  has  the  ap- 
pearance represented  in 
Fig.  196. 

It  seemh  probable  that 
primarily  the  mesoneph- 
ric  cords  an  arranged  segmentally,  a  single  pair  occurring 
in  each  segment  of  the  body  behind  the  pronephros  as  far 
back,  piobably,  astjie  pelvic  region,  and  hence  the  inter 
mediate  cell-mass  from  whicli  the  Wolffian  ridge  is  formed 
may  properly  be  regarded  as  composed  of  nephrotomes, 
even  though  no  surface  indications  of  segmentation  are  to 
be  seen  in  it .  The  correspondence  of  the  tubules  with  the 
myotomes  becomes,  however,  early  disturbed,  partly  as 
the  result  of  difi"erences  in  growth  of  the  tw^o  structures, 


Fio.  190.  -Tra.  'Rrsp  .Secttov  '<i- 
Tiifj  \\'()i.Fi-iA.\  KiiCK  OF  V  Chick 
Kmbrvo  or  TnKKE  Days. 

ai\  Aorta ;  ^l,  glonierulus ;  gr  genital 
ridge;  mrs,  mesentery;  mt,  mest>- 
iiepliric  tuhule;  vr,  cardinal  vein; 
IVd,  \V  -Iffian  duct.     (M'halkoiicz.) 


THE    MESON EPHKOS. 


365 


but  especially  because  a  number  of  secondary  and  tertiary 
tubules  develop  in  connection  with  each  of  the  primary 
ones.  Exactly  how  these  additional  tubules  arise  is  a 
little  uncertain,  some  observers  maintaining  that  they 
are  formcfl  from  the  substance  of  the  Wolffian  ridge  in  the 
same  manner  as  the  primary  tubules  with  which  they  later 


as 


Fio.  197.  rRiNi)(;iiNiT.\u  Api'.ak.vtis  ok  .\  M.\LK  I'li;  Kmbrvo  of  6  cm. 
an,   Aorta;  h,   bladder;    f^li,  Ruhernaculum  of    Hunter;    k,   kidney;    md, 

Miillerian  duct;  .cr,  suprarenal  body;  /,  testis;  w,  Wolffian  body;  wd, 

•  Wolffian  duct.  —(Milialkovic:.) 

become  connected  (Mihalkovicz),  while  others  hold  that 
they  are  formed  by  the  splitting  of  the  primary  tubules 
or  as  buds  from  these  (Braiiii,  Janhosik). 

By  the  formation  of  these  additional  tubules  and  the 
continued  elongation  of  all,  whereby  they  become  thrown 
into  numerous  convolutions,  the  Wolffian  ridge  becomes 
a  somewhat  voluminous  structure,  projecting  markedly 


366 


THE    DKVKLOl'MENT    OF   THE    HUMAN    UOUY. 


(If 


!l 


into  the  ca-lomic  cavity  (Fig.  197).  It  is  attached  to  the 
dorsal  wall  of  the  body  by  a  distinct  mesentery  and  has  in 
its  lateral  portion,  embedded  in  its  substance,  the  Wolffian 
duct,  while  on  its  mesial  surface  anteriorly  is  the  but 
slightly  developed  genital  ridge  (/).  This  condition  is 
reached  in  the  human  embryo  at  about  the  sixth  or 
seventh  week  of  development,  and  after  that  period  the 
mesonephros  undergoes  rapid  degeneration,  so  that  at 
about  the  sixteenth  week  nothing  remains  of  it  except  the 
duct  and  a  few  small  rudiments  whose  history  will  be 
given  later. 

The  Development  of  the  Metanephros.  —  The  meta- 
nephros  arises  as  an  outgrowth  from  the  dorsal  surface  of 
the  Wolffian  duct,  shortly  before  its  entrance  into  the 
cloaca  (Fig.  156).  The  outgrowth  is  of  a  tubular  form  and, 
as  it  elongates,  it  conies  to  lie  dorsal  to  the  mesonephros, 
its  anterior  end  enlarging  and  becoming  lobed  and  also 
becoming  surrounded  by  a  condensation  of  mesenchyme 
which  has  Ixren  termed  the  metanephric  blastema.  The 
outgrowth,  which  represents  the  ureter,  makes  its  appear- 
ance in  embryos  of  about  5  mm.,  but  its  anterior  end  does 
not  reach  itb  final  fx)sition  in  the  neighborhood  of  the 
suprarenal  body  until  the  third  month  of  development. 

The  de\>  lopment  of  the  tubules  of  the  metanephros 
has  been  studied  most  thoroughly  in  the  rabbit,  and  the 
description  which  follows  is  based  on  what  occurs  in  that 
animal.  The  extremity  of  the  ureter  early  begins  to 
branch  within  the  substance  of  the  blastema,  and  in  em- 
bryos of  twelve  days  it  has  given  rise  to  two  or  three 
branches  which  branch  again,  each  of  the  terminal 
!)ranches  ending  in  a  distinct  enlargement,  a  primary 
renal  vesicle,  which  lies  in  the  cortical  portion  of  the  blas- 
tema wlicli  l)y  tliis  time  has  formed  a  capsule  for  itself 
'  Fig.  if/r   Aj.     In  embryos  one  dav  older  each  of  the  renal 


f    i 

i 


THE    METANEPHKOS. 


367 


vesicles  has  given  rise  to  two  or  three  prolongations  which 
are  coiled  upon  themselves  in  an  S -shaped  manner  and 
represent  urinary  tubules  (Figs.  198,  B,  and  199,  A).  In 
what  would  correspond  with  the  lower  loop  of  the  S,  a 
collection  of  mesenchyme  appears  into  which  at  a  later 
stage  branches  penetrate  from  the  renal  artery,  producing 
a  glomerulus,  the  wall  of  the  tubule  in  this  region  becom- 
ing exceedingly  thin  to  form  a  capsule  of  Bowman  (Fig. 
199,  B,  he).  At  first  the  glomerulus  lies  close  to  the 
surface  of  the  kidney, 
but  as  development 
proceeds  it  is  gradu- 
ally carried  deeper 
into  the  cortical  por- 
tion by  the  elongation 
of  the  portion  of  the 
tubule  intervening  be- 
tween the  glomerulus 
and  the  primary  renal 
vesicle.  This  elonga- 
tion affects  at  first  the 
upper  limb  of  the  S, 
which  is  represented 
by  the  loop  of   Henle 

in  the  adult  kidney,  the  portion  between  the  loop  and 
the  glomerulus  forming  the  first,  and  that  between  the 
loop  and  the  renal  vesicle  the  second,  convoluted  tubule 
(Fig.  199,  C). 

In  the  mean  time  new  tubules  have  arisen  from  the 
vesicle  and  have  undergone  a  development  similar  to  what 
occurs  in  the  earlier  formed  ones,  and  the  formation  of 
new  tubules  continues  until  a  large  number  has  been  pro- 
duced from  each  renal  vesicle,  these  eventually  elongating 
to  form  the  collecting  tubules  (Fig.  199,  C).     Up  to  the 


Fk;.  198. —  DiACR.MUs  of  Early  Stages 
I.N  THE  Development  of  the  Meta- 

NEI'HRIC  TlTBULES. 

/,   Urinary  tubule;    Ur,  ureter;   v,  renal 
vesicle. — ( Haycrajt. ) 


368 


THE    DEVELOPMENT   OF    THE    HUMAN    BODY. 


it   i 


% 


H 


i     : 


1 1 


ill! 


time  when  the  urinary  tubules  begin  to  develop  there 
is  no  pelvis  to  the  kidney,  the  ureter  extending  well 
toward  the  center  of  the  blastema  before  beginning  to 
branch  and  the  branches  thence  extending  to- the  cortex 
(Fig  198).  As  soon  as  the  tubules  appear,  however,  the 
formation  of  the  pelvis  begins  by  what  has  been  described 
as  an  evagination  of  the  primary  branches  of  the  ureter  to 
form  a  common  cavity,  a  process  which  is  beginning  to 


^r   y 


F.r.      190.-THREE     STAGES     IN     THE     DEVELOPMENT    OF    A     Ur.NFEROUS 

TiBruE  OF  A  Rabbit. 
be,  Bownu.n-s  capsule;  fi,  glon.erulus;  /,    loop  of  H.nle;  r,  renal  ves,cle. 

-    {Uiiycrajt.) 

manifest  itself  in  the  stage  shown  in  Fig.  iq8,  B,  an.l 
which  is  continued  until  the  secondary  branches  are  also 
taken  up  into  the  cavitv,  into  which  the  various  collectmg 
tubules  then  open  separately. 

\t  about  the  tenth  week  of  development  the  surface  ol 
the  human  kid.icv  bcconies  marked  by  shallow  depressions 
into  lobes,  of  which  there  are  about  eighteen,  one  corre- 


THK    MLI.LI'.KIAN    DUCT. 


369 


spending  to  each  of  the  groups  of  tubules  which  arise  from 
the  same  renal  vesicle.  This  lobation  persists  until  after 
birth  and  then  disappears  completely,  the  surface  of  the 
kidney  becoming  smooth. 

From  what  has  been  said  above  it  will  be  seen  that  the  tubules 
of  the  metanephros  are  all  derived  from  the  original  outtjrowlh 
which  arises  from  the  Wolffian  dutt;  the  tissue  of  the  nieta 
nephric  blastema  give.:  rise  only  to  the  connective  tissue  and 
vessels  of  the  kidney.  It  was  at  one  time  maintained  that  thi' 
ureters  and  collecting  tubules  were  alone  develoj)ed  from  the 
outgrowth,  and  that  the  tubules  were  formed  independently  in 
the  blastema  and  only  later  united  with  the  collecting  tubules. 
The  view  presented  above  seems,  however,  to  more  nearly  repre- 
sent the  actual  processes  of  development. 

The  Development  of  the  Miillerian  Duct  and  of  the  Qeni° 
tal  Ridge. — At  the  time  wuen  the  Wolfilan  body  has  al- 
most reached  its  greatest  development  a  second  longitudi- 
nal duct  makes  its  appearance  in  close  proximity  to  the 
Wolffian.  This  is  known  as  the  Miillerian  duct  (Fig.  200, 
Md).  Its  development  is  preceded  by  the  appearance  of 
a  distinct  ridge  or  fold  upon  the  ventral  surface  of  the 
Wolffian  body,  extending  from  the  under  j,urface  of  the 
diaphragm  above  to  the  urogenital  sinus  below  and  con- 
taining in  the  lower  portion  of  its  course  the  Wolffian 
duct  (Fig.  197).  Near  the  anterior  end  of  the  mesoneph- 
ros  tliere  grows  into  this  fold  an  evagination  from  the 
peritoneum  covering  the  Wolffian  ridge  and  by  the  pro- 
liferation of  the  c<-lls  at  its  tip  this  evagination  gradually 
extends  downward  in  the  substance  of  the  ridge,  and  in 
embryos  of  22  mm.  has  reached  the  urogenital  sinus.  As 
they  approach  the  sinus,  the  right  and  left  evaginations 
or  Miillerian  ducts  gradually  approach  one  another  and 
finally  fuse  together  to  form  a  single  tube  in  the  lower  part 
of  their  course,  but  they  remain  distinct  al)Ove,  each  tube 
retaining  its  original  opening  iuto  the  peri'  oneal  cavity. 


1 


370 


TIIK    DKVEI.OI'MENT    OF     I'lIK    HUMAN    IIOOV. 


/ 


/ 


Fio.  200. —Transverse  Section  through  the  Abuominal  Re<;ion  of 

AN   KmBRYO  of  25   MM. 

Ao   Aorta;  /:?,  bladder;  /.intestine;    /..liver;  .U,  muscle;  .Uci.  Mullerian 
'   duct-  A',  spinal  cord;  Ov,  (jvary;  /^.4,  rectus  abdonnnis;  .S.i;,  spinal 
ganglion:  VA,  umbilical  artery;  Vr,  ureter;  r,  vertebra;  II  ,  W  olllian 
body;   IFii,  Wolffian  duct.— (ATeifce/.) 


THi:    (iKNllAI.    KIIUiK. 


37  « 


The  first  indication  of  the  appearance  of  the  genital 
ridge  is  the  assumption  of  a  hi'jh  columnar  form  by  the 
epithelial  cells  of  the  upper  part  of  the  mesial  surface  of 
the  Wolffian  ridge,  and  shortly  after  this  thickening  of  the 
epitheliutn  has  appeared  a  condensation  of  the  underlying 
mesenchyme  occurs  (I'ig.  194).     At  first  the  ridge  is  of 
insignificant  dimensions  compared   with  the  more  vol- 
uminous Wolffian  body,  but  as  the  degeneration  of  the 
latter  proceeds  the  relative  size  of  the  two  structures  be- 
comes reversed  and  the  genital  ridge  forms  a  marked 
prominence  attached  to  the  surface  of  the  Wolffian  ridge 
by  a  fold  of   peritoneum  which  becomes  the  mesovarium 
in  the  female  and  the  mcsorchium  in  the  male.     The  fold 
which  surrounds  the  Wolffian  body  becomes  transformed 
on  the  degeneration  of  that  structure  into  the  Imnul  luja- 
mcnt,  the  transverse  position  of  which  in  the  adult  is  due 
to  the  fusion  of  the  lower  portions  of  the  Miillerian  ducts, 
and  since  the  genital  ridges  lie  primarily  to  the  median 
side  of  the  ducts,  they  come  to  l)e  attached  by  their  mesen- 
tery to  the  dorsal  surface  of  the  broad  ligament.     The 
relations  of  the  broad  ligaments  and  mesorchia  in  the  male 
become  profoundly  modified  by  the  descent  of  the  testes 
into  the  scrotum,  a  process  to  be  described  later  (p.  388.) 

From  each  genital  ridge  a  prolongation  of  mesenchyme 
extends  downward  in  the  mesentery  of  the  ridge,  nearly 
parallel  with  the  Miillerian  duct,  with  which  it  comes  into 
contact  at  the  point  where  the  two  ducts  fuse  and  thence 
is  continued  downward  and  forward  between  the  folds  of 
the  broad  Ugament  to  be  attached  to  the  ventrrl  wall  of 
the  abdomen  in  the  inguinal  region^  The  upper  part  of 
this  prolongation  of  the  genital  ridge  represents  the  liga- 
ment of  the  ovary  diXvA  its  lower  part  the  %«'"t'"'^'^  teres  of 
the  female  (Fig.  201),  while  in  the  male  the  entire  struc- 
ture forms  what  is  known  as  the  ciuhcrnaculum  testis. 


1.0 


I.I 


if    1^ 


lu 


1^     M 


1.8 


1.4 


1.6 


MICROCOPY  RESOLUTION  TEST  CHART 

NATIONAL  BUREAU  OF  STANDARDS 

STANDARD  REFERENCE  MATERIAL  1010a 

(ANSI  and  (SO  TEST  CHART  No.  2) 


37 


TIIK    DKVKI.orMKNT    OK    TlIK    HUMAN     IIODV, 


V 


i-i 


Althousli  the  liistological  difterenliation  of  the  genital 
ridge  proceeds  along  similar  lines  in  both  sexes  until  about 
the  fifth  or  sixth  week,  it  seems  convenient  to  consider 
separately  the  entire  process  of  differentiation  as  it  occurs 
in  each  sex. 

The  Divclopment  of  the  Testis  —The  earliest  sign  of  de- 
velopment visible  in  the  testis  is  a  nniltiplication  of  the 

ejnthelial  cells  to  form  a  thick- 
layer,  into  the  under  surface 
of  which  deep  bays  extend 
from  the  subjacent  mesen- 
chynie,  producing  the  appear- 
ance of  cords  of  cells  extend- 
ing into  the  mesenchyme  from 
the  epithelium.  These  cords 
(Fig.  202,  ec)  consist  of  two 
kinds  of  cells,  (i)  elongated 
cells  with  a  small  amount  of 
protoplasm,  the  epithelial 
cells,  and  (2)  large  spherical 
cells  with  more  abundant, 
clear  protoplasm,  and  termed 
sexual  cells. 

While  tlie  development  is 
at  this  stage, — that  is,  at 
about  the  fourth  or  fifth 
week, — a  structure  makes  its 
appearance  which  serves  to  characterize  the  organ  as  a 
testis.  This  is  a  layer  of  connective  tissue  which  grows  in 
between  the  superficial  and  deep  layers  of  the  epithelium 
and  gradually  extends  around  the  entire  organ  to  form 
the  tunica  aUnujinca.  Shortly  after  its  appearance  the 
cords  of  cells  become  broken  up  into  more  or  less  spherical 
masses  by  the  growth  into  them  of  the  surrounding  mesen- 


-  kEI'ROlirCTIVE  <)r- 
(i.WS  OI-  A  I'e.mauK  Hmbkvo 
OK  Six  Months. 
/>',  lUaddcr;  /■',  I''all()i)ian  tube; 
/,  iiilesline;  I*/,  ovarian  liga- 
nient ;  ( 'r,  ovary;  A'/,  round 
li^'aim-nt ;  LA,  uniliilical  ar- 
tery; I'r,  ureter;  It,  uterus; 
ir,  W'ollVian  l)ody  (epooi)lio- 
rouK  {Adapttd  jrom  Milnil- 
koiicz  ) 


TlIK     IKS  lis. 


5/.'' 


I 


chyme,  and  into  th->  substance  of  the  testis  there  grow 
from  the  capsules  of  Bowman  in  the  neighboring  portions 
of  the  mesonephros  cords  of  cells  which  form  what  are 
known  as  the  medullary  cords  (I'ig.  202,  mc).  One  of  these 
cords  comes  into  relation  with  each  of  the  spherical  masses 
derived  from  the  epithelium,  and  the  cells  of  each  mass, 
both  the  epithelial  and  the  sexual,  arrange  themselves  m 
a  layer  surrounding  the  enclosing  wall  of  mesenchyme,  al- 


Vic,  202    -Section-  throi-.;h  the  Testis  and  tmk  Bkoad  I,i.;ament  of 

THE  Testis  ok  an  Kmbkvo  of  .i.i  mm. 

cc.  Kpithelial  cords;  e[>.  epitheli.un  ;  mr   luedullary  o.r.ls;  m./   MuUeriun 

duct;»n«,  mesorchiuinizrJ,  Womuin  duct.     {M.halko.nz.) 

though  no  lumen  as  yet  occurs  in  any  of  them.  This  con- 
dition is  reached  at  about  the  sixth  week,  and  from  this 
time  onward  until  the  approach  of  puberty,  the  changes 
which  occur  are  limited  to  a  growth  in  length  of  the  med- 
ullary cords  and  epithehal  masses,  lumina  not  appearing 
in  them  until  shortly  before  puberty. 

As  this  period  approaches  the  final  differentiation  of  the 
testis  is  completed.     A  lumen  appears  in  each  epithelial 


i'Sj 


m 


w 


174 


THK    DEVEI.OI'MKNT    OF    TIIK    HUMAN    BODV. 


mass,  which  thus  becomes  a  tubule,  and  the  medullary 
cords  are  also  transformed  into  tubules.  The  sexual  cells 
begin  to  multiply  and  assume  the  form  of  spermatogonia 
(see  p.  30),  while  the  epithelial  cells  become  transformed 
into  Sertoli  cells  (Benda).  There  is  some  difference  of 
opinion  as  to  whether  the  medullary  cords  take  any  part 
in  the  forniation  of  the  seminiferous  tubules  of  the  adult 
testis;  the  probability  seems  to  be  in  favor  of  the  view 
that  they  do  not,  the  seminiferous  tubules  being  derived 
from  the  epithelial  cords  alone,  while  the  medullary  cords 
give  rise  to  the  tubuli  recti  and  the  capsules  of  Bowman 
from  which  they  arise  to  the  rete  testis. 

The  Development  of  the  Ovary. — The  development  of  the 
ovary  starts  off  on  the  same  general  lines  as  that  of  the 
testis,  although  there  are  important  differences  in  the  de- 
tails. Two  distinct  elements  are  concerned,  as  in  the 
case  of  the  testis — namely,  cords  of  cells  derived  from  the 
epithelium  of  the  genital  ridge  and  prolongations  from  the 
uppermost  tubules  of  the  mesonephros;  but  the  relations 
of  the  two  elements  and  their  differentiation  are  very 
different  in  the  ovary. 

.The  ovarial  epithelial  cords  when  fully  developed  con- 
sist of  three  well-defined  portions:  (i)  a  lower  rather 
Cylindrical  portion,  (2)  an  intermediate  short  and  greatly 
thickened  portion,  and  (3)  an  outer  short  cylindrical  por- 
tion or  neck  (Fig.  203).  Each  cord  so  constituted  corre- 
sponds to  one  of  the  epithelial  cords  of  the  testis,  but 
whereas  in  the  latter  epithelial  and  sexual  cells  occur 
throughout  the  entire  length  of  the  cord,  in  the  ovary 
the  sexual  cells  are  found  only  in  the  intermediate  en- 
largement. At  an  early  stage  the  lower  cylindrical  portions 
of  the  ovarial  cords  become  separated  from  the  interme- 
diate portions  by  connective  tissue  and  form  what  have 
been  termed  the  medullary  cords,  though  it  is  clear  that 


rur.  ovAKV. 


375 


they  arc  not  homologous  with  the  structures  so  named  in 

the  testis. 

After  this  separation  the  intermediate  portion  of  eaeli 

cord  is  penetrated  by  bands  ot 
mesenchyme  in  such  a  maimer 
that  it  becomes  divided  into 
secondary  cyhndrical  cords 
known  as  Pfliujcr's  cords  (Fig. 
204),  and  these  latter  again  be- 
come divided  transverselv  into 
rounded  masses,  the  Graafian 
jolliclcs,  each  of  which  contains, 
as  a  rule,  but  a  single    sexual 


ImC.     20,^.  — DiACiRAM      oi- 

AN  Hpithelial  In- 
vagination OP  THK 
Ovary  ok  a  Rabbit. 
(■/>,  Ovarial  e])itheliutn  ;  r, 
intermediate  enlarge- 
ment ccmtaininR  germ 
cells;  /,  proximal  cyl- 
indrical portion ;  mr, 
medullary  cord. — {von 
Winiuaiicr.) 


ric.  204.  SkcTion  ok  THK  Ovary  OK  a 
New-born  Ciiild. 

a  Ovarial  epithelium;  /'.  proximal  part 
'  of  an  epithelial  cord ;  c,  germ  cell  m 
epithelium;  (/,  intermediate  enlarge- 
ment of  an  epithelial  cord;  e,  group 
of  gtnn  cells  enclosed  in  a  follicle; 
7  single  germ  cells  with  follicles; 
g,  blood-vessel.— (/-Vt-w  (k-ginhaiir,  ajhr 
Waldcycr.) 


cell  which  is  enclosed  within  a  mass  of  epithelial  cells, 
the  whole  being  surrounded    by  a  condensed  zone  of 


I 


«■:.•« 


I'm 


i 


376 


TIIK    DKVKI.OI'MKNI-    Ol"     Till:    HUMAN    ItODV. 


mesenchyme,  wliicli  eventually  becomes  richly  vascular- 
ized and  forms  the  thvca  jolliculi  (iMg.  9).  The  epithe- 
lial cells  in  each  follicle  are  at  first  comi)aratively  few  in 
number  and  closely  surround  the  sexual  cell  (iMg.  204,  c) 
which  is  destined  to  become  an  ovum,  but  in  certain 
of  the  follicles  they  underi^o  an  increase  by  mitosis,  be- 
coming extremely  numerous,  and  laicr  secrete  a  fluid,  the 
liquor  folliculi,  which  collects  at  one  side  of  the  follicle 
and  eventually  forms  a  considerable  portion  of  its  con- 
tents. The  follicular  cells  are  differentiated  by  its  ap- 
pearance into  tile  stratum  (jranulosum,  which  surrounds 
the  wall  of  the  follicle,  and  the  discus  proligctus,  in  which 
the  ovum  is  embedded  (I'ig.  9,  dp),  and  the  cells  which 
immediately  surround  the  ovum,  becoming  cylindrical 
in  shape,  give  rise  to  the  corona  radiata  (Fig.  to,  cr). 

The  elements  derived  from  the  mesonephros  which 
correspond  to  the  medullary  cords  of  the  testis  do  not 
reach  as  extensive  a  development  as  in  that  organ  and, 
indeed,  do  not  really  penetrate  into  the  substance  of  the 
ovary,  but  form  a  network,  the  rctc  ovarii,  lying  in  the 
mesovarium  along  the  line  of  its  junction  with  the  ovary. 
In  some  mammals,  such  as  the  rabbit,  they  come  into 
contact  with  the  so-called  ovarian  medullary  cords,  the 
similarity  to  the  conditions  obtaining  in  the  testis  lieing 
thus  greatly  increased. 

The  Transformation  of  the  Mesonephros  and  the  Ducts. 
— At  one  period  of  development  there  are  present,  as  rep- 
resentatives of  the  urinogenital  apparatus,  the  Wolffian 
body  (mesonephros)  and  its  duct,  the  Miillerian  duct,  and 
the  developing  ovary  or  testis.  Such  a  condition  forms 
an  indifferent  stage  from  which  the  development  proceeds 
in  one  of  two  directions  according  as  the  genital  ridge  be- 
comes I  testis  or  an  ovary,  the  Wolffian  body  in  part 
undergoing  degeneration  and  in  part  persisting  to  form 


TIIK    TRANSFORMATION    OF    TIIK    MI:S()M:1M1K0S. 


377 


orj^ans  which  for  the  most  part  arc  rudimentary,  while  in 
the  female  the  Wolflian  duct  also  deitenerates  except  for 
certain  rudiments  and  in  the  male  the  Miilleriaii  duct 
behaves  similarly. 

In  the  Male.— It  has  been  seen  that  the  upper  portion 
of  the  Wolffian  body,  in  giving  rise  to  the  medullary  cords 
of  the  testis,  enters  into  very  intimate  relations  witli  that 
organ  and  may  be  regarded  as  divided  into  two  portions, 
an  upper  genital  and  a  lower  excretory.     In  the  male  the 
genital  portion  of  the  body  persists  in  Hs  entire^  v,  serving 
as  the  efferent  ducts  of  the  testis,  which,  beginnmr  in  the 
spaces  of  the  rete  testis,  already  shown  to  represent  the 
capsules  of  Bowman,  open  into  the  upper  pan  of  the  Wolf- 
fian duct  and  form  the  globus  major  of   the  epididymis. 
The  excretory  portion  undergoes  extensive  degeneration, 
a  portion  of  it  persisting  as  a  mass  of  coiled  tubules  ending 
blindly  at  both  ends,  situated  near  the  head  of  the  epididy- 
mis and  known  as  the  paradidymis  or  oiyan  of  Giraldes, 
while  a  single  elongated  tubule,  arising  from  the  portion 
of  the  Wolffian  duct  which  forms  the  globus  minor  of  the 
epididymis,  represents  another  portion  of  it  and  is  known 
as  the  vas  aberrant'. 

The  Wolffian  duct  is  retained  complete,  the  portion  of 
it  nearest  the  testis  becoming  greatly  elongated  and 
thrown  into  numerous  coils,  forming  the  body  and  globus 
minor  of  the  epididymis,  while  the  remainder  of  it  is  con- 
verted into  tlie  vas  deferens  and  the  ductus  ejaculatorius. 
A  lateral  outpouching  of  the  wall  of  the  duct  to  form  a 
longitudinal  fold  appears  at  about  the  third  month  and 
gives  rise  to  the  vesicula  scminalis,  the  lateral  position  of 
the  outgrowth  explaining  the  adult  position  of  the  vesi- 
cula? lateral  to  the  vasa  deferentia. 

With  the  Miillerian  duct  the  case  is  very  difTcrent,  since 
it  disappears  completely  throughout  the  greater  part  of  its 
32 


ill 


.'  1  > 

hi- 


378 


THE    OEVEI-OPMENT    OF    THE    HUMAN    BODY. 


course,  only  its  upper  and  lower  ends  persisting;,  ilic 
former  K'ivinj;  rise  to  a  small  sac  like  body,  the  sessile 
hydatid  of  Motijacjui,  attached  to  the  upper  end  of  the 


A'  - 


ID 


IM 


rCMALC 


INDIFFERENT 


MALF 


Fin.    205 — Diagrams    Illtstkatint,    thk    Tkansformations    of    the 

MULLERIAN  AMI    Wdl.ll  IAN   DlCTS. 

B,  Bladder;  C,  clitoris;  CG,  canal  <.|"  C'.acrtncr;  ( '/,  cloaca;  Eo,  cpo- 
o])lioron;  £>,  epididymis;  /  ,  I''all()pian  tiihc;  G",  genital  ^land ;  ///•;, 
hydatid  of  epididymis;  //.M,  hydatid  of  MorgaRiii;  A',  kidney;  MP, 
Miillerian  duct;  (',  ovary;  /',  penis;  l\>,  i)aro()])lioron ;  I'r,  prostate 
Rland;  A",  rectum;  T,  testis;  I',  urethra;  ^  A/,- uterus  masculinus; 
Ir,  ureter;  T-S,  urogenital  sinus;  L't,  uterus;  \\  vagina;  I'.-l,  vas 
aberrans;  17),  vas  deferens;  V'.S",  vesicula  scminalis;  W'H,  Wolffian 
body;  II7>,  Wolffian  (Xuci.  -{Modified  from  Ifiixhy.) 

testis  near  the  epididymis,  while  the  latter  is  represented 
by  a  depression  in  the  floor  of  the  urethra  known  as  the 


rilK     IK  \\>I'()KM.\II<>\    (»K    nil.    MKSOM  ltll«». 


3/"'^ 


//•; 


>i  I 


n> 


r.v 

A' 


sinus  pocuhnis,  wliidi  is  usually  prolonged  upward  into  a 
short  cvliudrical  pouch  known  as  the  uterus  nmscuUtius, 
thouijh  it  corresponds  to  the  vaijina  rather  than  to  the 
uterus  of  the  female. 

///  the  Femnlc.  In  the  female  the  i^enital  portion  of  the 
mesonephros,  thou.ijh  never  functional  as  ducts,  persists 
as  ;i  1,'roup  of  ten  to  fifteen  tubules,  situated  between  the 
two  layers  of  the  broad  li^^ament  and  in  close  proximity  to 
the  ovary;  these  constitute  what  i;  known  as  the  cpo- 
dpliityon  (panminum  or  onjiin  oj  Roseumidlcr).  The  tu- 
bule •  •  '  Hndly  at  the  ends  nearest  the  ovary,  but  at  the 
otht  acre  they  are  somewhat  coiled,  they  open  into 

a  c(    «  ciuct  which  rejiresents  the  upper  end  of  the 

Woli duct       Xear  this  rudimentary  body  is  another, 

also  composed  of  tubules,  representing,'  the  remains  of  the 
excretory  portion  of  the  mesonephros  and  termed  the 
parodplioron.  So  far  as  the  mesonephros  is  concerned, 
tlierefore,  the  persisting  rudiments  in  the  female  arc  com- 
parable to  those  occurriuij  in  the  male. 

As  ret,'ards  the  ducts,  however,  the  case  is  dilTerent,  for 
in  the  female  it  is  the  Miillerian  ducts  which  persist,  while 
the  Wolftians  underi^o  det^eneration,  a  small  portion  of 
their  upper  ends  persistinti;  in  coimection  with  the  epo- 
ophora,  while  their  lower  ends  persist  as  straight  tubules 
Iving  at  the  sides  of  the  vagina  and  forming  what  are 
known  as  the  canals  oj  (idttncr.  The  Miillerian  ducts,  on 
the  other  hand,  become  converted  into  the  Fallopian 
tubes,  and  in  their  lower  portions  into  the  uterus  and 
vagina.  From  the  margins  of  the  openings  by  whic  the 
Mullerian  ducts  communicate  with  the  ccelom  projections 
develop  at  an  early  period  and  gi\e  rise  to  the  fimhrite, 
witli  the  exception  of  the  one  connected  with  the  ovary, 
the  fimbria  ovariai,  which  is  the  upper  persisting  portion 
of  the  original  genital  ridge,  its  lower  portion,  below  the 
ovary,  being  represented  by  the  ovarian  and  inguinal 


ILf' 


,  I 


3S0 


THE    DEVELOPMENT    OF     TlfE    HUMAN    BODY. 


ligament  already  described.  It  has  been  seen  that  the 
lower  portions  of  the  Mullerian  ducts  fuse  together  to 
form  a  single  canal,  and  it  is  from  this  that  the  uterus  and 
vagina  are  differentiated,  the  histological  distii  ction  of  the 
two  portions  commencing  to  manifest  itself  at  about  the 
third  month.  Durmg  the  fourth  month  the  vaginal  por- 
tion of  the  duct  becomes  flattened  and  the  epitlielium 
lining  its  lumen  fuses  so  as  to  completely  occlude  it  and, 
a  little  later,  there  appears  near  its  lower  opening  a  dis- 
tinct semicircular  fold  attached  to  its  dorsal  margin.  This 
is  the  hymen,  a  structure  which  seems  to  be  represented  in 
ihe  male  by  the  veru  montanum.  The  obliteration  of  the 
lumen  of  the  vagina  persists  until  about  the  sixth  month, 
when  the  cavity  is  re-established  by  the  breaking  down 
of  the  central  epithelial  cells. 

The  diagram,  Fig.  205,  illustrates  the  transformation 
from  the  indifferent  condition  which  occjirs  in  the  two 
sexes,  and  that  the  homologies  of  the  various  parts  may  be 
clearly  understood  they  may  also  be  stated  in  tabular 
form  as  follows : 


Indifferent  Stage. 


Mai.k. 


Genital  ridge, *  Tt^t\s. 

{  Gubernaculum. 

(Globus  major  of 
..„ _^,  epididymis. 

1  Paradidymis. 

\  Vasa  aberrantia. 

/  Body  and  globus  mi 

Wolffian  ducts \       "or  of  epididymis 

j  Vasa  deferentia. 
'  Ejacul-*  Dry  ducts. 

f  Sessile  nv^atid. 

Mullenan  ducts, J  i 

( I  Uterus  mascuHnus. 


\ 


Female. 

Fimbria  ovarica. 
Oviry. 

Ovarian  ligament. 
Round"  ligament. 

Epodphoron. 
Paroophoron. 


Collecting  tubules  of 
epodphoron. 

Canals  of  Gartner. 

Fallopian  tubes. 

Uterus. 

Vagina. 


THF.  lu.vnnKK. 


.^«l 


In  addition  to  the  sossilc  hydatid,  a  stallccd  hydaiul  also  oicurs 
in  connection  with  the  testis,  and  a  similar  structnre  is  attached 
to  the  fimbriated  opcninjj  of  each  I'allopian  tnhe.      The  sijjjnifi- 
cance  of  these  strnctures  is  nncertain,  though  it  has  been  suk' 
gested  that  they  arc  persisting  rudiments  of  the  pronephros. 

A  failure  of  the  developmeiit  of  the  various  parts  just  de 
scrihed  to  he  completed  in  the  normal  manner  leads  to  various 
abnormalities  in  connection  with  the  reproductive  organs. 
Thus  there  may  occur  a  failure  in  the  fusion  of  the  lower  por- 
tions of  the  Miillerian  ducts,  a  bihorned  or  bipartite  uterus  re- 
sulting, or  the  two  ducts  may  come  into  contact  and  their  adja- 
cent walls  fail  to  disappear,  the  result  being  a  median  partition 
separatintT  e  vagina  or  both  the  v:ii;ina  and  uterus  into  two 
coinpartnn  j.  The  excessive  development  of  the  fold  which 
gives  rise  to  the  hymen  may  lead  to  a  complete  closure  of  the 
lower  opening  <.i  the  vagina,  while,  on  the  other  hand,  a  failure 
of  the  Miillerian  ducts  to  fuse  may  produce  a  biperforate 
hvmen. 


The  Development  of  the  Urinary  Bladder  and  the  Uro- 
);enitai  Sinus. — So  far  the  relations  of  the  lower  ends  ol  the 
urino^renital  ducts  havr  not  been  considered  in  detail,  al- 
though it  has  been  seen  that  in  the  earl/  stages  oi  develop- 
ment the  Wolffian  and  »itillcrian  ducts  open  into  the  sides 
of  the  ventral  portion  of  the  cloaca;  that  the  ureters  com- 
municate with  the  lower  poitions  of  the  Wolffian  ducts; 
that  from  the  ventral  anterior  portion  of  the  cloaca  the 
allantoic  duct  extends  outward  into  the  belh -stalk;  and, 
finally  fp.  29 7  \  that  the  cloaca  becomes  di^ded  into  a 
dorsal  portion,  which  forms  tjie  lower  part  of  the  rectum, 
and  a  ventral  portion,  which  is  continuous  with  the  allan- 
tois  and  receives  the  urinogenital  ducts  (Fig.  206).  It  [•> 
the  history  of  this  ventral  portion  of  the  cloaca  which  is 
now  to  be  considered. 

It  may  be  regarded  as  consisting  of  two  portions,  an 
anterior  and  a  posterior,  the  line  of  insertion  of  the  urino- 
genital ducts  marking  the  junction  of  tiit- two  The  ante- 
rior or  upper  portion  is  destined  \'  j^ive  rise  to  the  urinary 


m 


im 


m 


W^^^T. 


1 


38: 


TiiK  r)Kvi:r.()i'MKNr  or   tiik  lit  man  hodv. 


MaddtT  (I'V  206./)).  whik.  tlu'  Unwr  oiu-  forms  what  is 
known  for  a  tiim- as  the- urogenital  sinii«,  is,,).  Tlu- Maddtr. 
wlion  first  dilTiTcntiati'd.  is  a  tubular  structure,  whose  lu- 
men is  continuou  ith  that  of  the  alluntois.  hut  after  the 
second  month  it  enlirj^es  to  become  more  sac  like,  while 
the  intra  embryonic  portionof  the  allantois  degenerates  to 
a  solid  cord  extendin.t,' from  the  apex  of  the  bladder  to  the 


I"l<;.    206,       RlXOXbTKl  CTION    ,  ir    TIIK    Cm.ACAI.    Ri-i.K.N-    ,,f    ax     I'MBKVO 

OI"    14   MM. 

al.  AllunK.is:  /,.  hhuhler;  ,</,  genital  tuhcrck-;  /.   inUstini';   „,  spinal  curd 
"<•,  nutc.cliurd:  r.  recluiii;  vj;,  urinuKcnital  sinus;  (//,  urcltT- 
nan  duct.     (Kiibcl.) 


a',  Wolf- 


umbilicus  and  is  known  as  the  umchus.  Durinj^  the  en- 
largement of  the  bladder  the  terminal  portions  of  the 
urinoirenital  ducts  become  taken  up  into  its  walls,  a  pro- 
cess which  continues  until  finally  the  ureters  and  Wolffian 
ducts  open  into  it  separa*  -.  the  ureters  opening  to  the 
sides  of  and  a  little  anteri^i  to  the  ducts.  This  condition 
is  reached  in  embryos  of  about  14  mm.  (Fig.  206),  and  in 


inK  I'Ki  riiKA. 


v^,^ 


latir  stai,'is  tlu'  ijitcrxal  l)ct\vi'i'ii  tin-  two  pairs  of  ducts 
isincrt-asfd  '  Im.U  -"7  '•  re-'.:  inK  «»  tlu-  (■ornuitioti  of  a  short 
canal  (onmrtintj  tlu-  lower  (.nd  of  the  bladder  wliicli  re 
Cfivcs  tlu-  urfltrs  with  the  upiur  itid  of  tlu-  iirotjinital 
sinus,  into  which  the  W'oltliati  .md  MulUriaii  ducts  open. 
Tliis  connect  in-;  canal  represeiiis  the  untln.t  ( I"ii;.  207, 
ur),  or  rather  the  entire    urethra  of  the  female  and  the 


ff 


•  i. 


y*--        —  - 


:•> "    -  7 


/■ 


I'h.     207         RlXoN-TKl  CTIOX    or    Till-:    Cl.OACAl,    Stki  CTIKKS    or    AN     V.St- 

HKVO  t>l"  2r<   MM. 

W,  Uladdor:  m.  Miilli'ri.m  duel;  r.  recUiin;  vu.  un.trenital  sinus;  vi,  syin- 
plivsis  i)ul)is;  //,  uritor;  in,  uR'lhra;  a.-,  Wnlllian  duci.  -(A, l,i[^hd 
jrom  K,  ihJ.) 

proximal  part  of  that  of  the  male,  since  a  considerable 
portion  of  the  latter  canal  is  still  undeveloped  (see  p.  3S6). 
Krom  this  urethra  there  is  developed,  at  about  the  third 
month,  a  scries  of  solid  longitudinal  folds  which  project 
upon  the  outer  surface  and  separate  from  the  urethra  from 
above  downward.     These  represent  the  tubules  of  the 


¥m 


m 


384 


THE    DEVELOPMENT   OK    THE    HUMAN    BODY. 


prostate  gland  and  are  developed  in  both  sexes,  although 
they  remain  in  a  somewhat  rudimentary  condition  in 
the  female.  The  muscular  tissue,  so  characteristic  of  the 
gland  in  the  adult  male,  is  developed  from  the  surrounding 
mesenchyme  at  a  later  stage. 

The  urogenital  sinus  is  in  the  early  stages  also  tubular 
in  its  upper  part,  though  it  expands  considerably  below, 
where  it  is  closed  by  the  cloacal  membrane.  This,  by  the 
separation  of  the  cloaca  into  rectum  and  sinus,  has  be- 
come divided  into  two  portions,  the  more  ventral  of  which 
closes  the  sinus  and  the  dorsal  tlie  rectum,  the  interval  be- 
tween them  having  become  considerably  thickened  to 
form  the  perineal  body.  In  embryos  of  about  1 7  mm.  the 
urogenital  portion  of  the  membrane  has  broken  through, 
and  in  later  stages  the  tubular  portion  of  the  sinus  is 
gradually  taken  up  into  the  more  expanded  lower  portion, 
until  finally  the  entire  sinus  forms  a  shallow  depression, 
termed  the  vestibule,  into  the  upper  part  of  which  the  ure- 
thra opens,  while  below  are  the  openings  of  theWolffian 
(ejaculatory)  ducts  in  the  male  or  the  orifice  of  the  vagina 
in  the  female.  From  the  sides  of  the  lower  part  of  the 
sinus  a  pair  of  evaginations  arise  toward  the  end  of  the 
fourth  month  and  give  rise  to  the  glands  of  Bartholin  of  the 
female  or  the  corresponding  Cowper's  glands  in  the  male. 

The  Development  of  the  External  Genitalia.— At  about 
the  fifth  week,  before  the  urogenital  sinus  has  opened  to 
the  exterior,  the  mesenchyme  on  its  ventral  wall  begins  to 
thicken,  producing  a  slight  projection  to  the  exterior.  This 
eminence,  which  is  known  as  the  genital  tubercle  (Fig.  206, 
tjt),  rapidly  increases  in  size,  its  extremity  becomes  some- 
what bulbously  enlarged  (Fig.  208,  gl)  and  a  groove,  ex- 
tending to  the  base  of  the  terminal  enlargement,  appears 
upon  its  vestibular  surface,  the  lips  of  the  groove  forming 
two  well-marked  genital  folds  (Fig.  208,  gf).     At  about  the 


m 


THE    EXTEKNAI.    GENITALIA. 


385 


tenth  week  there  appears  on  either  side  of  the  tubercle  an 
enlargement  termed  the  genital  swelling  (Fig.  208,  gs), 
which  is  due  to  a  thickening  of  the  mesenchyme  of  the 
lower  part  of  the  ventral  abdominal  wall  in  the  region 
where  the  inguinal  ligament  is  attached,  and  with  the  ap- 
pearance of  these  structures  the  indifferent  stage  of  the 
external  genitals  is  completed. 

In  the  female  the  growth  of  the  genital  tubercle  pro- 
ceeds rather  slowly  and  it  becomes  transformed  into  the 
clitoris,  the  genital  folds  becoming  prolonged  to  form  the 


Fk;.  208.— The  External  Genitalia  of  an  Embryo  of  25  mm. 
a,  Anus;  g/,  genital  fold;  gl,  glans;  g.f,  genital  swelling;  p,  perineal  body. 

—{Keihel.) 

labia  minora.  The  genital  swellings  increase  in  size,  their 
mesenchyme  becomes  transformed  into  a  mass  of  adipose 
and  fibrous  tissue  and  they  become  converted  into  the  labia 
major  a,  the  interval  between  them  constituting  the  vulva. 
In  the  male  the  early  stages  of  development  are  closely 
similar  to  those  of  the  female;  indeed,  it  has  been  well 
said  that  the  external  genitals  of  the  adult  female  resemble 
those  of  the  fetal  male.  In  early  stages  the  genital  tuber- 
cle elongates  to  form  the  penis  and  the  integument  which 
covers  the  proximal  part  of  it  grows  forward  as  a  fold 


J' 
I 

i 

I 

r<        I    i 


miM 


386 


THE    DEVELOPMENT    OF    THE    HUMAN    BODV. 


which  encloses    the    bulbous  enlargement  or  qlans  and 
forms  the  prepuce,  whose  epithelium  fuses  with  that  cover- 
ing the  glans  and  only  separates  from  it  later  by  a  cornifi- 
cation  of  the  cells  along  the  plane  of  fusion.     The  genital 
folds  meet  together  and  fuse,  converting  the  vestibule 
and  the  groove  upon  the  vestibular  surface  of  the  penis 
into  the  terminal  portion  of  the  male  urethra  and  bring- 
ing  it   about   that   the   vasa   defcrentia  and  the  uterus 
masculinus  open  upon  the    floor  of    that  passage.     The 
two  genital  swellings  are  at  the  same  time  brought  closer 
together,  so  as  to  lie  between  the  base  of  the  penis  and 
the  perineal  body  and,  eventually,  they  unite  together 
to  form  the  scrotum,  the  line  of  their  junction  being  indi- 
cated by  the  median  raphe.     The  mesenchyme  of  which 
they  were  prunarily  composed  differentiates  into  the  same 
layers  as  are  found  in  the  wall  of  the  a!)domen  and  a  peri- 
toneal pouch  is  prolonged  into  them  from  the  abdomen, 
so  that  they  form  sacs  into  which  the  testes  descend  to- 
ward the  close  of  fetal  life  (see  p.  388). 

The  homologies  of  the  portions  of  the  reproductive 
apparatus  derived  from  the  cloaca  and  of  the  external 
genitalia  in  the  two  sexes  may  be  perceived  from  the 
following  table : 


•'     :,! 


Malk. 


Urogenital  sinus, 
Genital  tubercle, 
Genital  folds,  .  . 
Genital  swellinj^s, 


Urinary  bladder. 

I  Proximal    portion   of   ure- 
i       thru. 

Cowper's  glands. 

The  rest  of  the  urethra. 

Penis. 

Prepuce. 

Scrotum. 


j  Fkmai.e. 

Urinary  l)ladder. 
Urethra. 

Glands  of  Rartholin. 
X'estibule. 
Clitoris, 
Labia  minora. 
Labia  niajora. 


It  is  Stated  above  that  the  layers  which  compose  the  walls  of 


TMK    DKSCKNT    OK    TIIF.    OVAKIK.S. 


5^7 


the  scrotum  an-  identical  with  those  of  the  abdominal  wall. 
This  mav  he  seen  in  detail  from  the  following  scheme: 


AnnoMiNAi    Wai.i.s. 
IlUCKUt'lcnt. 
Superficial  fasci:i 
Kxternal  ohH<iiu-  tmisde. 
Internal  ol;'iJ|iie  muscle. 
Transversalis  nuisde. 
Peritoneum. 


SCROIIM. 

Integument. 
Dart  OS. 

Intercdlumnar  fascia. 
Cremasteric  fascia. 
Infundihulifnrm  fascia. 
Tunica  vaj{inalis. 


Xumerous  anomalies,  depending  upon  an  inhibition  ()r  excess 
of  the  development  of  the  parts,  may  occur  in  ccmneetion  with 
the  external  genitalia.     vShould,   for  instance,  the  lips  of  the 
groove  on  the  vestibular  surface  of  the  penis  fail  to  fuse,  the 
penial  portion  of  the  urethra  remains  incomplete,  constituting 
a  condition  known  -is  /!;'/>« v/xfi/ai.v,  a  condition  which  offers  a 
serious  bar  to   the  fulfilment  of  the  sexual  act.     If  the  hypo- 
spadias is  complete  and  there  be  at  the  same  time  an  imperfect 
development  of  the  penis,  as  frequently  occurs  in  such  cases, 
the  male  genitalia  closely  resemble  those  of  the  female  and  a 
condition  is  produced  which  is  usually  known  as  hcnnaphrodil- 
ism.     It  is  noteworthv  that  in  such  cases  there  is  frecjuently  a 
somewhat  excessive  development  of  the  uterus  masculinus,  and 
a  similar  condition  mav  be  produced  in  the  female  by  an  ex- 
cessive  development    of    the   clitoris.     Such    cases,    however, 
which  concern  only  the  accessory  organs  of  reproduction,  are 
instances  of  what  is  more  properly  termed  spurious  hermaphro- 
ditism, true  hermai)hroditism  being  a  term  which  should  be  re- 
served for  possible  cases  in  which  the  genital  ridges  give  rise  m 
the  same  individual  to  both  ova  and  spermatozoa.     vSuch  cases 
are  of  exceeding  raritv  in  the  human  species,  although  occa- 
sionallv  observed  in  the  lower  vertebrates,  and  the  great  major- 
ity of  "the  examples  of  hermaphroditism  hitherto  observed  are 
cases  of  the  spurious  variety. 

The  Descent  of  the  Ovaries  and  Testes.— The  positions 
finally  occupied  by  the  ovaries  and  testes  are  very  differ- 
ent from  those  which  they  possess  in  the  earlier  stages  of 
development,  and  this  is  especially  true  in  the  case  of  the 
testes.  The  change  of  position  is  partly  due  to  the  rate  f 
growth  of  the  inguinal   ligaments  being  less  than  that 


; 


1 


v 

1' 

1  ;• 

V 

. , : 

■i 

388 


THE    DEVELOPMENT   OF   THE    HUMAN    BOnV. 


«  ,1 

I 

1 

I 


of  the  abdominal  walls,  the  reproductive  organs  being 
thereby  drawn  downward  toward  the  inguinal  regions 
where  tlie  ligaments  are  attached.  The  attachment  is  to 
the  bottom  of  a  slight  pouch  of  peritoneum  which  projects 
a  short  distance  into  the  substance  of  the  genital  swellings 
and  is  known  as  the  canal  of  Nuck  in  the  female,  and  in 
the  male  as  the  vaginal  proceu. 

In  the  female  a  second  factor  con  ibines  with  that  just 
mentioned.  Tlie  relative  shortening  of  the  inguinal  liga- 
ments acting  alone  would  draw  the  ovaries  toward  the 
inguinal  regions,  but  the  fusion  of  the  lower  ends  of  the 
Mullerian  ducts,  since  the  inguinal  ligaments  are  united 
with  these  (see  p.  371),  produces  a  traction  toward  the 
median  line,  so  that  the  organs  come  to  lie  finally  in  the 
true  pelvis. 

With  the  testes  the  case  is  more  complicated,  since  in 
addition  to  the  relative  shortening  of  the  inguinal  liga- 
ments, there  is  an  elongation  of  the  vaginal  processes  into 
the  substance  of  the  genital  swellings.     Three  stages  may 
be  recognized  in  the  descent  of  the  testes.     The  first  of 
these  depends  on  the  slow  rate  of  elongation  of  the  ingui- 
nal ligaments  or  gubernaculum.     It  lasts  until  about  the 
fifth  month  of  development,  when  the  testes  lie  in  the  in- 
guinal region  of  the  abdomen,  but  during  this  month  the 
elongation  of  the  gubernaculum  becomes  more  rapid  and 
brings  about  the  second  stage,  during  which  there  is  a 
slight  ascent  of  the  testes,  so  that  they  come  to  lie  a  little 
higher  in  the  abdomen.     This  stage  is,  however,  of  short 
duration,  and  is  succeeded  by  the  stage  of  the  final  de- 
scent, which  is  characterized  by  the  elongation  of  the  vagi- 
nal processes  of  the  peritoneum  into  the  substan  j  of  the 
scrotum  fFig.  209,  A).     Since  the  gubernaculum  is  at- 
tached to  the  bottom  of  the  process,  and  since  its  growth 
has  again  diminished,  the  testes  gradually  assume  again 


THE    HESCENT    OF    THE    TESTES. 


389 


their  inguinal  position,  and  are  finally  drawn  down  into 
the  scrotum,  slipping  down  between  the  walls  of  the  vagi- 
nal processes  and  the  infv  .abuliform  fascia,  hich,  to- 
gether with  the  other  layers  composing  the  scrotal  wall, 
are  differentiated  at  about  this  time. 

The  condition  which  is  tin?  acquired  persists  for  some 
time  after  birth,  the  testicles  being  readily  pushed  up- 
ward into  the  abdominal  cavity  along  the  cavity  by  which 
they  descended.     Later,  however,  the  size  of  the  openings 


Fif,   209.— Diagrams  Illustrating  the  Descent  of  the  Testis. 
il.  Inguinal  ligament;  w,  muscular  layer;  s,  skin  and  dartos  of  the  scro- 
tum; /,  testis;  tv,  tunica  vaginalis;  rd,  vas  deferens;  vp,  vagmal  pro- 
cess of  peritoneum. — (After  Hertwig.) 

of  the  vaginal  processes  into  the  general  peritoneal  cavity 
becomes  greatly  reduced,  so  that  each  process  becomes 
converted  into  an  upper  narrow  neck  and  a  lower  sac  ake 
cavity  (Fig.  209,  B),  and,  still  later,  the  walls  of  the  neck 
portion  fuse  and  become  converted  into  a  solid  cord,  while 
the  jower  portion,  wrapping  itself  around  the  testis,  be- 
comes the  tunica  vaginalis  (tv).  By  these  changes  the 
testes  become  per^  %nently  located  in  the  scrotum.  Dur- 
ing their  descent  the  testes  are  drawn  downward  out  of 
the  Tnesenteries,  the  mesorchia,  in  which  they  were  origi- 


li 


M 


390 


THE    DEVELOPMENT   OF   THE    HUMAN    BODV, 


nally  enclosed,  and  these  structures  flatten  out  and  dis- 
appear, and,  since  the  remains  of  each  Wolffian  body,  the 
epididymis,  and  the  upper  part  of  each  vas  deferen-  to- 
gether with  the  spermatic  ver.sels  and  nerves,  are  drawn 
down  into  the  scrotum  with  each  testis,  ihe  mesenterial 
fold  comparable  to  the  broad  lija^ament  of  the  female  also 
practically  disappears,  becoming  converted  into  a  sheath 
of  connective  tissue  which  encloses  the  vas  deferens  and 
the  vessels  and  nerves,  binding  them  together  into  what 
is  termed  the  spermatic  cord. 

In  the  textbooks  of  anatomy  the  spernialic  cord  is  usually 
described  as  lying  in  an  inguinal  canal  which  traverses  the 
abdominal  walls  obliquely  immediatelv  above  Poupart's  liga- 
ment. vSo  long  as  the  lumen  of  the  neck  portion  of  the  vaginal 
process  of  peritoneum  remains  patent  there  is  such  a  canal, 
placmg  the  cavity  of  the  tunica  vaginalis  in  ccnmnmieatiori 
with  the  general  peritoneal  cavit  -,  but  the  cord  does  not  traverse 
this  canal  but  lies  outside  it  in  th^  retroperitoneal  connective 
tissue.  When,  however,  the  neck  of  the  vaginal  process  disap- 
pears, a  canal  no  longer  exists,  although  the  connective  tissue 
which  surrounds  the  spermatic  cord  and  unites  it  with  the 
tissues  of  the  abdominal  walls  is  less  dense  than  the  neighboring 
tissues,  so  that  the  cord  may  readilv  be  separated  from  these 
and  thus  appear  to  lie  in  a  canal. 

The  Development  of  the  Suprarenal  Bodies.— The  supra- 
renal bodies  make  their  appearance  at  an  early  stage, 
while  the  Wolffian  bodies  are  still  in  a  well-developed  con- 
dition, and  they  are  situated  at  first  to  the  medial  side  of 
the  upper  ends  of  these  structures  fFig.  197,  sr).  Their 
final  relation  to  the  metanephros  is  a  secondary  event,  and 
in  merely  a  topographic  relation,  there  being  no  develop- 
mental relation  bet  •  een  the  two  structures. 

Their  developn  .1  has  been  very  variously  described. 
In  the  Mannnalia  they  arise  by  the  proliferation  of  cells 
situated  at  the  extremities  of  invaginations  of  the  calomic 
epithelium  into  the  Wolffian  ridge  (Fig.  210),  the  groups 


in 


THE  SUPRARENAL  BODIES, 


391 


of  cells  so  formed  from  the  several  invaginations  later  unit- 
ing together  to  fo.  m  a  re.atively  large  organ.  The  invagi- 
nations resemble  closely  in  appearance  and  position  the 
tubules  and  funnels  of  the  pronephros  (sec  p.  361),  and 
they  have  recently  f  Aichel)  been  regarded  as  representing 
funnels  belonging  to  the  mesonephros.  One  of  the  char- 
acteristics of  the  mammalian  mesonephros  is  that  it  pos- 
sesses no  nephrostomes,  but  in  the  lower  vertebrates  such 
structures  do  occur,  and  it  is  possible  that  the  inv:i  Jna- 
tions  of  the  coelomic  epithelium  which  give  rise  to  the 
suprarenals    may   be    representatives   of   certain    meso- 


ffC 


Ao 


'^\ 


Fig.  210.— Section  throlijh  .\  Portion  of  the  Woli-ki.w  RiofiE  of 

.\  R.\BBIT  KmBRYO  of  6."   MM. 

Ao,  Aorta;     ns,  nephrostome ;   Sr,  supraren       body;    re,  cardinal  viir. ; 
uc,  tubule  of  Wolffian  body;  ivd,  Wi.-.Juin  duct.— (.4ic/je/.) 

nephric  funnels  which  have  failed  to  unite  with  the  tu- 
bules and  have  undergone  a  secondary  transformation. 

That  the  suprarenals  are  primarily  connected  with  tlie 
mesonephros  becomes  exceedingly  probable  from  the  fact 
that  similar  structures,  known  as  the  accessory  suprarenals 
of  Marchand,  not  infrequently  occur  between  the  layers 
of  the  broad  ligament  of  the  female  and  in  the  vicinity  of 
the  epididymis  in  the  male  and  are  developed  from  the 
degenerating  tubules  of  the  epoophoron  or  paroophoron, 
and  presumablv  from  the  corresponding  structures  in  the 
male. 


392 


THE    DEVELOPMENT   OF   THE    HUMAN    HODY. 


It  is  doubtful,  however,  if  the  entire  mass  of  the  supra- 
renal organs  is  derived  from  the  constituents  furnished  by 
the  mesonephros,  although  this  is  the  view  maintained  by 
the  most  recent  investigator  of  the  subject  (Aichel).  In 
the  fully  formed  organs  a  clear  distinction  obtains  between 
the  cortical  and  medullary  portions,  and  earlier  observers 
very  generally  tnaintained  that  the  latter  was  derived 
from  cells  which  separated  from  the  neighboring  ganglia 
of  the  sympathetic  nervous  system.  Strong  support  is 
afforded  to  this  view  by  the  close  connections  which  exist 
between  the  organs  and  the  sympithetic  nervous  system 
in  the  adult  condition,  and  also  by  the  fact  that  the  cells 
of  the  medullary  substance  possess  a  strong  affinity  for 
chromium  salts,  assuming  a  distinctly  brown  color  when 
treated  with  solutions  of  these  salts.  The  same  chrom- 
affine  nature  is  characteristic  of  the  cells  of  certain  other 
organs,  such  as  the  intercarotid  ganglia  and  Zuckcr- 
kandl's  organs  (see  p.  450J,  whose  origin  from  the  sym- 
pathetic system  seems  to  be  beyond  question. 

It  is  probable,  therefore,  that  the  suprarenal  organs  are 
formed  by  a  combination  of  two  constituents,  one  of 
which,  derived  from  the  mesonephros,  forms  the  cortical 
portion  of  the  organs,  while  the  other,  having  its  origin 
from  the  sympathetic  ganglia,  gives  rise  to  the  medullary 
portion.  The  mesenchyme  in  the  vicinity  of  each  organ 
condenses  around  it  to  form  a  capsule,  and  the  organs  in 
later  stages  receive  a  rich  blood-supply. 


LITERATURE. 

O.  Aichel:  " Vergleichende  Entwickelungsgescliichte  iind  vStamtticsgc- 
schichte  der  Nebennieren,"  Arcliiv  jtir  mikrosk.  A.uit.,  uvi,  1<)00. 

O.  Franku:  "Beitriige  zur  Lehrevom  Descensus  testiculoruni,"  Siizungs- 
ber.  dis  kais.  Akad.  Wisscnsch.  Wun,  Maih.-Naturuiss.  Clu-^r,  cix, 
1900 


tlTEKMUKb:. 


393 


J.  B.  Havcrakt:  "The  Develupment  of  ilie  Kidney  in  the  Rablnt,"  Intir- 

nat.  Monahschrift  fur  Anal,  und  Physiol.,  xii,  1898. 
J.  Janosik:    "Histi)logisch-einl)ry('loxisclie     Unlersuchunj;en     iiber    das 

Urogenitalsysteni,  "Silzun^shir.    dvr   kni.f.  .\kad.    W'issinscb.    IVien, 

Math.-Xalurwiss.Clasu-,  xci,  1887. 

F.  Keibel;  "Zur  Entwickelungsgeschiclite  des  nienschlichen  I'rogenital- 

apparatus,"  Archiv  fur  Anal,  und  Physiol.,  .\nal.  Ablh.,  1896. 
J.   B.  M.\CAI,uum:     "  Notes  on  the  Wolflian  Body  of  Higher  Manuiials," 

Amer.  Journ.  of  Anal.,  i.   1902. 
K.  Martin:  "Ucber  die  Anlagc  der  Urnicre  beini  Kaninchen,"  Archiv.  jiir 

Anal,  und  Physiol.,  Anal.  Ablh.,  1888. 
H.  Meyer:  "Die  Entwickelung der  Urnieren  beini  Menschen,"  Archiv.  jiir 

mikrosk.  Anal.,  xxxvi,  1890. 

G.  VON  MiHALKOvicz:  "  Untersuchungen  iiber  die  Hntwickehing  des  Harn- 

und  Geschlechtsapparates  der  Anmiolen,"  Inkrnat.  Monalsschrijt  jitr 
.\naf.  und  Physiol.,  ir,  1885. 

^  '.  X.\(;eu:  "Ueber  die  Kntwickclung  dts  Urogenitalsystenis  des  Mensch- 
en," Archiv  jiir  mikrosk.  Anal.,  xxxiv,  18h'^. 

\V.  N.xc.EL:  "Ueber  die  Entwickelung  des  Uterus  und  der  Vagina  beim 
Menschen,"  Archiv  jur  mikrosk.  Anal.,  xx.wii,  1891. 

W.  X.\OEU:  "Ueber  die  Entwickelung  der  innere  und  iiussere  Genitalien 
beim  menschhchen  Weiber,"  Archiv  jiir  GynakoK,  xuv,  1894. 

G.  Pailin:  "Beitriige  zur  Anatomie  der  Prostata  und  der  Sunienblasen," 
Archiv  jiir  Anut.  und  Physiol.,  Anal.  Abth.,  1901. 

A.  SoLu6:  "Sur  la  migration  des  Testicules,"  Complcs  Rcndus  ..  la  .*■" 
de  liiol.  Paris,  Ser  lOme,  ii,  1895. 

A.  Sorui^:  " Sur  le  mecanisme  de  la  migration  des  testicules,"  Com/'/fi- 
Rcndus  dc  la  Soc.  de  Biol.  Paris,  Ser  lOme,  ii,  1895. 

F.  TouRNEfx:  "Sur  le  developpement  et  revolution  du  tubercule  genital 
chez  le  fa?tus  humain  dans  les  deux  sexes,"  Journ.  dc  I'Anat.  et  de  la 
Physiol,  XXV,  1889. 

S.  Weber:  "Zur  Entwickelungsgeschichte  Jes  uropoetischen  Apparates 
bei  Siiugem,  mit  besonderer  Berucksichtigung  der  Urniere  zur  Zeit 
des  Auftretens  der  bleibenden  Niere,"  Morphol.  .Arbcitcn,  vii,  1897. 

P.  WendELEr:  "Die  fotale  Entwickelung  der  menschhchen  Tuben,"  Ar- 
chiv. jiir  mikrosk.  Anal.,  XLV,  1895. 

H.  VON  Winiwarter:  "Recherches  sur  I'ovogen^se  et  I'organogenese  de 
I'ovaire  des  Mammifferes"  (Lapin  et  Homme),  Archives  de  Biol.,  xvii, 
1900. 


ill 


Ml 


33 


CHAPTKR  XIV. 

THE  DEVELOPMENT  OF  THE  NERVOUS 

SYSTEM. 

The  Histogenesis  of  the  Nervous  System.— The  entire 
central  nervous  system  is  derived  from  the  cells  lining  the 
medullary  groove,  whose  formation  and  conversion  into 
the  medullarv  canal  has  already  been  described  (p.  1 14)- 
■/hen  the  groove  is  lirst  formed,  the  cells  lining  it  are 
somewhat  more  columnar  in  shape  than  those  on  either 
side  of  it,  though  like  them  they  are  arranged  in  a  single 
layer;  later  thev  increase  by  mitotic  division  and  arrange 
themselves  in  several  layers,  so  that  the  ectoderm  of  the 
groove  becomes  verv  much  thicker  than  that  of  the  gene- 
ral surface  of  the  body.     While  its  tissue  is  in  this  condi- 
tion the  lips  of  the  groove  unite,  and  the  subseciuent  differ- 
entiation of  the  canal  so  formed  differs   somewhat    in 
diflferent  regions,  although  a  fundamental  plan  may  be 
recognized.     This  plan  is  most  readily  perceived  in  the 
region  which  becomes  the  spinal  cord,  and  may  be  de- 
scribed as  seen  in  tliat  region. 

Throughout  the  earlier  stages,  the  cells  lining  the  inner 
wall  of  the  medullary  tube  are  found  in  active  prolifera- 
tion, some  of  the  cells  so  produced  arranging  themselves 
with  their  long  axes  at  right  angles  to  the  central  canal 
and  extending  throughout  the  entire  thickness  of  the 
wall  to  form  a  supportive  framework  (Fig.  211),  while 
others,  whose  destinv  is  for  the  most  part  not  yet  deter- 
minable, and  whicli  therefore  may  be  termed  itidifjcreut 
cells,  occur  in  the  meshes  of  this  framework.     At  this 

394 


Tiir  iiisio(;i:nksis  or   riir.  m-.kvous  sv^ikm. 


395 


It 


staijc  a  transverse  section  of  the  medullary  tube  shows 
to  be  composed  of  two  well  defined  zones,  an  inner  one 
immediatelv  surrounding'  the  central  canal  and  composed 
of  the  indilTerent  cells  and  the  bodies  of  the  supportive 
or  ifycmlrmil  nils,  and  an  outer  one  consistini;  of  the 
branched   prolonijations  of   the   ependymal   cells.     This 
outer  layer  is  termed 
the     mnrqinnl     rcluni 
(Randschleier)       (V'l^. 
211,  nil).     The   indif- 
ferent cells  now  bejfin 
to  wander  outward  to 
form  a  definite  layer, 
termed      the      mantle 
layer,    lyin.i;    l-"tween 
tlie     murs^inal     volum 
and  the  boflies  of  the 
ependymal  cells   ( I'ii^. 
2  12),   and    when    this 
layer  has  become  well 
established    the    cells 
composing  it  be,i,Mn  to 
divide  and  to  difTeren- 
tiate     into     ( i  )     cells 
termed        muiohlasis, 
destiiied     to     become 
nerve-cells,     and     (2) 
others    which    appear 
to    be   supportive    in 
cliaracter  and  are  termed   neuroglia  cells  (Fij^^.  212,  B). 
The  latter  are  for  the  most  part  small  and  have  their 
cell-bodies  drawn  out  into  very  numerous  and  exceed- 
ingly slender  processes,  which  ramify  among  the  neuro- 
blasts, these,  on  the  other  hand,  being  larger  and  each 


ic  211.     I'j'KNDVMAi,  Cells  from  tub 
Si'iNAL  Cord  of  an   Hmbrvo  of  4.25 

MM. 

niv.  Marginal  velum.— (///v.) 


396 


THE    r>F.VKI.OPMKNT   OF   THK    HUMAN    IJODV. 


early  dcvcbping  a  sin^'lc  strong  process  which  grows 
out  into  the  marginal  velum  and  is  known  as  an  axis- 
cylinder.  At  a  later  period  the  neuroblasts  also  give  rise 
to  other  processes,  ter  ncd  dendrites,  more  slender  and 
shorter  than  tlie  axis-cylinders,  branching  repeatedly  and, 
as  a  rule,  not  extending  beyond  the  limits  of  the  mantle 
layer. 

The  axis    ylinder  prwesses  of  the  majority  of  the  neuro- 


Fic.    2!  2.— Diagrams  showinc,   the    Develoi-ment   of   the    Mantle 

Layer  ly  :he  Simnau  Cord. 
The  circles,  indifferent  cells;  circles  with  dots,  neuroglia  cells;  shaded  cells, 

germinal  cells:   circles  with  cross,  germinal   cells  in  mitosis;   black 

cells,  nerve-cells. — {Schaper.) 

blasts  on  reaching  the  marginal  velum  bend  upward  or 
downward  and,  after  traversing  a  greater  or  less  length 
of  the  cord,  re-enter  the  mantle  layer  and  terminate  by 
dividing  into  numerous  short  branches  which  come  into 
relation  with  the  dendrites  of  adjacent  neuroblasts.  The 
processes  of  certain  cells  situated  in  the  ventral  region 
of  the  mantle  zone  pass,  however,  directly  through  the 


v..>ii." 


i  I 


TIIK    IIIVKMiKNIvSIS   OF    THK    NKKVOUS    SVSTKM. 


397 


niar«tiial  vtUun  out  into  the  surrounding  tissues  and  con- 
stitute the  viutnil  ncnc  roots  (h\.  215). 

The  dopsal  nerve  loots  have  a  very  different  origin.     In 
eml)ryos  of  about  2.5  mm.,  in  which  the  medullary  canal 
is  only  partly  closed  (Fig.  42),  the  cells  which  lie  along 
the  line  of  transition  between  the  lips  of  tiie  grcx)ve  and 
the  general  ectoderm  form  a  distinct  ru\^v  readily  recog- 
nized    in    sections    and 
termed  the   ticnral  riihje 
(Fig.  2  1,^.  \).   When  the 
lips  of   the   groove   fuse 
together  the  cells  of  the 
crest   unite    to    form    a 
wedge  shaped         mass, 
completing    the    closure 
of   the   canal   (Fig.    21.^. 
B),  and  later  proliferate 
so  as  to  extend  outward 
over  the  surface  of  the 
canal      (Fig.      213,     C). 
Since    this   proliferation    '"^ 
is  most  active  in  the  re-      -^oOg 
gions  of  the  crest  v.iiicli 
correspond  to  the  meso- 
dermic   somites  there  is 
formed  a   series  of    cell 
masses,     arranged     scg- 
mentally  and  situated  in  the  mesenchyme  at  the  siiles 
of  the  medullary  canal  (Fig.  200).  These  cell-masses  rep- 
lesentthe  postenor  root  ganglia,  and  certain  of  their  con- 
stituent cells,  which  may  also  be  termed  neuroblasts,  early 
assume  a  fusiform  shape  and  send  out  a  process  from  each 
extremity.     One  of  the.se  processes,   the  axis-cylinder, 
grt    s  inward  toward  the  medullary  canal  and  penetrates 


'OlO 


C 


Fid.  2l,V — TiiKEE  Sections  throich 
THE  .Medullary  Canal  ok  an  Km- 
BRYO  OK  2..S  MM. — (I'OM  Lctihossck.) 


t 


11; 


398 


THE    DEVELOPMENT    OF   THE    HUMAN    BODY. 


its  marj^Miial  velum,  and,  after  a  longer  or  shorter  course  in 
this  zone,  enters  the  mantle  layer  and  comes  into  contact 
with  the  dendrites  of  some  of  the  central  neuroblasts.  The 
other  process  extends  peripherally  and  is  to  be  regarded  as 
an  extremely  elongated  dendrite.  The  processes  from  the 
cells  of  each  ganglion  aggregate  to  form  a  nerve,  that 
formed  by  the  axis-cylinders  being  the  posterior  root  of  a 
spinal  nerve,  while  that  formed  by  the  dendrites  soon 
unites  with  the  ventral  nerve-root  of  the  corresponding 
segment  to  form  the  main  stem  of  a  spinal  nerve. 


i 


V 


\'l',.      214.       CiCLI.S     IKOM     THK     C'.ASSIvKIAX     ('..\X<  ll.ll  ).\     OF     A     ClINIJA    I'll. 

K.MBkVO 

((,   bipolar   cell ;  /i  and  r.  transitional    staijes   ti)   (/,  'i"  shaped   cells,      {viiii 

(iihuchti  n.) 

There  is  thus  a  \ery  important  difference  in  the  modes 
of  development  of  the  two  nerve-roots,  the  axis-cylinders 
of  the  ventra  )ots  arising  from  cells  situated  in  the  wall 
of  the  medullary  canal  and  growing  outward  (centrifu- 
gally  ),  while  those  of  the  dorsal  root  spring  from  cells  situ- 
ated i)eripherally  and  grow  inward  (centripetally)  toward 
the  medullary  canal.  In  tlie  majority  of  the  dorsal  root 
ganglia  the  points  of  origin  of  tlie  two  processes  of  each  bi- 
polar cell  gradually  approach  one  another  and  eventually 


TMK    HISTOGKNKSIS    OI-     TlIK    NKKVOIS    SVSTKM. 


399 


come  to  arise  from  a  common  stem,  a  process  of  the  cell- 
body  which  thus  assumes  a  characteristic  T  form. 

From  what  has  been  said  it  will  be  seen  that  each  axis- 
cvlinder  is  an  outgrowth  from  a  single  neuroblast  and  is  part  of 
its  cell  bodv,  as  are  also  the  dendrites.  Consequently  there  is 
strong embrvological  evidence  in  support  of  the  murom-  theory, 
which  regards  the  entire  nervous  system  as  composed  of  sepa- 
rate units,  each  of  which  corresponds  to  a  cell  and  is  termed  a 
neurone.  Doubts  have  recently  been  thrown  on  the  complete 
individualitv  of  the  neurones  in  the  adult  (Apathy.  Bethe),  but 
both  from  the  embryological  and  physiological  standpoints 
their  primary  distinction  seems  to  be  established. 

By  the  development  of  the  axis-cylinders  which  occupy 
the  meshes  of  the  marginal  velum,  that  zone  increases  in 
thickness  and  comes  to  consist  principally  of  nerve-fibers, 
while  the  cell-bodies  of  the  neurones  of  the  cord  arc  situ- 
ated in  the  mantle  zone.  Xo  such  definite  distinction  of 
color  in  the  two  zones  as  exists  in  the  adult  is,  however, 
noticeable  until  a  late  period  of  development,  the  medul- 
lary sheaths,  which  give  to  the  nerve-fibers  their  white 
appearance  not  beginning  to  appear  until  the  fiftli  month 
and  continuing  to  form  from  that  time  onward  until  after 
birth.  The  origin  of  the  myelin  which  composes  the 
medullary  sheaths  is  as  yet  uncertain,  although  the  more 
recent  observations  tend  to  show  that  it  is  picked  out  from 
the  blood  and  deposited  around  the  axis-cylinders  in  some 
manner  not  yet  understood.  Its  appearance  is  of  im- 
portance as  being  associated  with  the  beginning  of  the 
functional  activity  of  the  nerve-fibers. 

Various  theories  have  been  advanced  to  account  for  the 
formation  of  the  medullary  sheaths.  It  has  been  held  that  the 
nivelin  is  formed  at  the  expense  of  the  outermost  portions  of 
the  axis  cylinders  themselves  (von  Kolliker),  and,  on  the  other 
hand,  it  has  been  regarded  as  an  excretion  of  the  cells  which 
compose  the  primitive  sheaths  surrounding  the  fibers  (Ranvier), 
a  theorv  which  is.  however,  invalidated  bv  the  fact  that  myelin 


m 


II 


400 


THE    DEVELOPMENT    OK    THE    HUMAN    BODY. 


is  formed  around  the  fibers  of  the  central  nervous  system  which 
possess  no  primitive  sheaths.  As  stated  above,  the  more  recent 
observatiotis  (Wlassak)  indicate  its  exogenous  origin. 

It  has  been  seen  that  the  central  canal  is  closed  in  the 
mid-dorsal  line  by  a  mass  of  cells  derived  from  the  neural 
crest.  These  cells  do  not  take  part  in  the  formation  of 
the  mantle  layer,  but  become  completely  converted  into 
ependymal  tissue,  and  the  same  is  true  of  the  cells  situ- 
ated in  the  mid-ventral  line  of  the  canal.  In  these  two 
regions,  known  as  the  roof-plate  and  ftoor-platc  respec- 
tively, the  wall  of  the  canal  has  a  characteristic  structure 
and  does  not  share  to  any  great  extent  in  the  increase  of 
thickness  which  distinguishes  the  other  regions  (Fig.  215). 
In  the  lateral  walls  of  the  canal  there  is  also  noticeable  a 
ditlerentiation  into  two  regions,  a  dorsal  one  standing  in 
relation  to  the  ingrowing  libers  from  the  dorsal  root  gan- 
glia and  known  as  the  dorsal  zone,  and  a  ventral  one,  the 
ventral  zone,  similarly  related  to  the  ventral  nerve-roots. 
In  different  regions  of  the  medullary  tube  these  zones,  as 
well  as  the  roof-  and  floor-plates,  undergo  different  de- 
grees of  development,  producing  peculiarities  which  may 
now  be  considered. 

The  Development  of  the  Spinal  Cord.— Even  before  the 
lips  of  the  medullary  groove  have  met  a  marked  enlarge- 
ment of  the  anterior  portion  of  the  canal  is  noticeable,  the 
region  which  will  become  the  brain  being  thus  distin- 
guished from  the  more  posterior  portion  which  will  be 
converted  into  the  spinal  cord.  When  the  formation  of 
tlie  mesodermic  somites  is  completed,  the  spinal  cord  ter- 
minates at  the  level  of  the  last  somite,  and  in  this  region 
still  retains  its  connection  with  the  ectoderm  of  the 
dorsal  surface  of  the  body ;  but  in  that  portion  of  the  cord 
which  is  posterior  to  the  first  coccygeal  segment  the  his- 
tological differentiation  does  not  proceed  beyond  the  stage 


TlIK    SPINAL    C(»K1>. 


401 


when  the  walls  consist  of  several  layers  of  similar  cells; 
the  formation  of  neuroblasts  and  nerve-roots  ceasing  with 
the  -.gnient  named.     After  tlie  fourth  month  the  more 
dilTerentiated  portion  elongates  at  a  much  slower  rate 
than  the  surrounding  tissues  and  so  appears  to  recede  up 
the  spinal  canal,   until   its  termination  is  opposite  the 
second  lumbar  vertebra.     The  less  dilTerentiated  portion, 
which  retains  its  connection  with  the  ectoderm  until  about 
the  fifth  month,  is,  on  the  other  hand,  drawn  out  into  a 
slender  filament  whose  cells  degenerate  during  the  sixth 
month,  except  in  its  uppermost  part,  so  that  it  comes  to 
be  represented  throughout  the  greater  part  of  its  extent 
by  a  thin  cord  composed  of  pia  mater.     Thi    cord  is  the 
structure  known  in  the  adult  as  the  filum  tcrminale,  and 
lies  in  the  center  of  a  leash  of  nerves  occupying  the  lower 
part  of  the  spinal  canal  and  termed  the  catida  equina. 
The  existence  of  the  cauda  is  due  to  the  recession  of  the 
cord  which  necessitates  for  the  lower  lumbar,  sacral  and 
coccygeal  nerves,  a  descent  through  the  spinal  canal  for  a 
greater  or  less  distance,  before  they  can  reach  the  inter- 
vertebral foramina  through  which  they  make  their  exit. 

In  the  early  stages  of  development  the  central  canal  of 
the  cord  is  quite  large  and  of  an  elongated  oval  form,  but 
later  it  becomes  somewhat  rhomboidal  in  shape  (Fig.  215, 
A),  the  lateral  angles  marking  the  boundaries  between  the 
dorsal  and  ventral  zones.     As  development  proceeds  the 
sides  of  the  canal  in  the  dorsal  region  gradually  approach 
one  another  and  eventually  fuse,  so  that  this  portion  of 
the  canal  becomes  obliterated  (Fig.  2r5,  B)  and  is  indi- 
cated by  the  dorsal  longitudinal  fissure  in  the  adult  cor-', 
the  central  canal  of  which  corresponds  to  the  ventral  por- 
tion onlv  of  the  embryonic  cavity.     While  this  process  has 
been  going  on  both  the  roof-  and  the  floor-plate  have  be- 
come depressed  below  the  level  of  the  general  surface  of 
34 


m 


U 


r' 


I) 


402 


THE    DKVELOPMENT    OF    THE    HUMAN    liOOY. 


the  cord,  and  by  a  continuance  of  the  depression  of  the 
floor-plate — a  process  really  due  to  the  enlargement  and 
consequent  bulging  of  the  ver/ral  zone — the  ventral  fis- 
sure is  produced,  the  difference  between  its  shape  and  that 
of  the  dorsal  fissure  being  due  to  the  difference  in  its 
development. 


m 


Vu..  21  >.    -Tkansvkksk  Sixtions  TiiKorr.ii  the  Si'ixal  Cokhs  ok  Ivm- 

BKVCS    OF    (.1)    ABOUT    FoLK    AXI)    A    HaUi-     WIvUKS    AND    (/>')     ABOCT 

TiiREK  Months. 
cH,  Column  of   Hurdacli;  cf,",  column  of  C.oll;  (//;,  dorsal  liorn;  Jr,  dorsal 
/one;  }fy,  noor-i)late;  ah,  oval  imndk-;  r/),  roof-plate;  rli,  ventral  horn; 
t:,  ventral  zone. — (///v.) 

The  development  of  the  mantle  layer  proceeds  at  first 
more  rapidly  in  the  ventral  zone  than  in  the  dorsal,  so  that 
at  an  early  stage  (Fig.  215,  A)  tli  ■  anterior  horn  of  gray 
matter  is  much  more  pronounced,  but  un  the  ;ievclopmcnt 
of  the  dorsal  nerve-roots  the  formation  of  neuroblasts  in 
the  dorsal  zone  proceeds  apace,  resulting  in  the  formation 


THE    SPINAL    CORD. 


403 


of  a  dorsal  horn.  A  small  portion  of  tlic  zone,  situated 
between  the  point  of  entrance  of  the  dorsal  nerve-roots 
and  the  roof-plate,  fails,  however,  to  j^ive  rise  to  neuro- 
blasts and  is  entirely  converted  into  ependynui.  This 
represents  the  future  column  of  Goll  (Fi^.  220,  A,  cG),  and 
at  the  point  of  entrance  of  the  dorsal  roots  into  the 
cord  a  well-marked  oval  bundle  of  fibers  is  formed  (Fig. 
215,  A,  ob)  which,  as  development  proceeds,  creeps  dor- 
sally  over  the  surface  of  the  dorsal  horn  until  it  meets  the 
lateral  surface  of  the  column  of  Goll,  and,  its  further  pro- 
gress toward  the  median  line  being  thus  impeded,  it  in- 
sinuates itself  between  that  column  and  the  posterior  horn 
to  form  the  column  of  Burdach  fFig.  :?i5,  B,  cB). 

Nothing  definite  is  as  yet  known  concerning  the  development 
of  the  other  columns  which  arc  rf^cognizabk-  in  the  adult  cord, 
but,  from  what  is  known  of  the  adult  anatomy,  it  seems  certain 
that  the  crossed  and  pyramidal  tracts  are  comp')scd  of  fibers 
which  grow  downward  in  the  meshes  of  the  marginal  vehmi 
from  neuroblasts  situated  in  the  cerebral  cortex,  while  the  c'"  "ct 
cerebellar  tract  and  the  fibers  of  the  ground  bundles  have  their 
origin  from  cells  of  the  mantle  layer  of  the  corck 

The  myelination  of  the  fibers  of  the  spinal  cord  begins  be- 
tween the  fifth  and  sixth  months  and  appears  first  in  tlif  col 
umns  of  Burdach,  and  about  a  month  later  in  thecolunuis  of 
Goll.  The  myelination  of  the  great  motor  paths,  the  crossed 
and  direc*  '•amidal  tracts,  is  the  last  to  develop,  appearing 
tow  ard  t  1  of  the  ninth,  month  of  fetal  life. 

The  Development  of  the  Brain. — The  enlargement  of  the 
anterior  portion  of  the  medullary  canal  docs  not  take  place 
quite  uniformly,  but  is  less  along  two  transverse  lines  than 
elsewhere,  so  tliat  the  brain  region  early  becomes  divided 
into  three  primary  vesicles  which  undergo  further  differen- 
tiation as  follows.  Upon  each  side  of  the  anterior  vesicio 
an  evagination  appears  and  becomes  converted  into  a 
club-shaped  structure  attached  to  the  ventral  portion  of 
the  vesicle  by  a  pedicle.     These  evaginations  (Fig.  216, 


m 


404 


THE    nEVELOI'MENT   OI"    THE    HUMAN    nODY. 


1    : 


i 
t    1 


op)  are  known  as  the  optic  cvaqinations,  and  beinj?  con- 
cerned in  the  formation  of  the  eye  will  be  considered  in 
the  succeedinj;  chapter.  After  their  formation  the  antero- 
lateral portions  of  the  vesicle  become  bulged  out  into  two 

protuberances  {h)  which  rap- 
idly increase  in  size  and  give 
rise    eventually    to    the    two 
cerebral     hemispheres,     which 
form,  together  with  the  por- 
tion of  the  vesicle  which  lies 
between       them,      what       is 
termed    the    telencephalon    or 
fore-brain,   the    remainder  of 
the  vesicle  giving  rise  to  what 
is  known  as  the  diencephrlon 
(thalamencephalon)  or '/  }een- 
brain  (Kig.  216,  /).     The  mid- 
dle vesicle  is  bodily  converted 
into  the  mesencephalon  or  mid- 
brain  (m),  but    the   posterior 
vesicle   dilTerentiates  so  that 
three    parts    may    be    recog- 
nized:   (i)    a    rather   narrow 
portion    which     immediately 
succeeds    the    mid-brain   and 
is    termed    the    isthmus    (/); 
(2)  a  portion  whose  roof  and 
floor  give  rise  to  the  cerebel- 
lum   and    pons    respectively, 
and  which  is  termed  the  mctcncphalon  or  hind-brain  {mt) ; 
and  (3)  a  terminal  portion  which  is  known  as  the  medulla 
oblongata,  or,  to    retain  a    consistent  nomenclature,  the 
myelencephalon  or  after-brain  (my).     From  each  of  these 
six  divisions  delmite  structures  arise  whose  relations  to 


l'"li..  210.  RlX'oNSTKreTloN"  Ol' 
TlIK  Bk.MN  Ol-  AN  IvMHKVO  Ol" 
2.15  MM. 

//,  Hvtiiispliere;  /,  istlumis;  w, 
incscnct'plKiloii ;  nij,  iiiid-bniiii 
tlcxurc,  )"/,  nietencipliahin; 
mv,  iiivck-ncepli:ili)ii;  »/,  neck 
tlexure;  at,  Dlic  capstilc;  nf^, 
(iptic  t'vaj,Mnatinii:  /,  lluiki- 
nicnce])lial(in.  -  {His.) 


THE    HRAIN. 


405 


the  secondary  divisions  and  to  the  primary  vesicles  may 
be  understood  from  the  followinj;  table  and  froi "  the 
annexed  figure  (Fig.  217),  which  represents  a  median  lon- 
gitudinal section  of  the  brain  of  a  fetus  of  three  months. 


/Myelcncephaltin 


Medulla  (il)liin>;;ita  (I). 


Isl  \'esiolc, 


2d  Vesicle, 


3d  Vesicle, 


Metenceplialon 
Istlinuis 

Mesencephalon 
Dienccjjlialon 

Telencejjlialon 


r<)ns(ll  1). 

Certhelhitii  (112). 

Superinr     peduncles     of      the 

cercl)elluni(m). 
Crura  cerebri    (posterior  jior- 

tion), 
r  Crura    cerebri  (anterior   por- 
I      tion)  (IV  1). 
(•Corpora  quadrigeniina  (I\'  2). 
/- I'ars  nianniiillaria(\'  1). 
I  ()]itic  thalanms  (V  2). 
iKpiphysis(V  .^). 

/  Infundilmluni  (VI  1). 
I  Cor])HS  striatum  (VI  2). 

Olfactory  hull)  (Vi.1). 

Hemispheres  (\'I  4). 


But  while  the  walls  of  the  primary  vesicles  undergo  this 
complex  differentiation,  their  cavities  retain  much  more 
perfectly  their  original  relations,  only  that  of  the  first 
vesicle  sharing  to  any  great  extent  the  modifications  of 
the  walls.  The  cavity  of  the  third  vesicle  persists  in  the 
adult  as  the  tourtli  ventricle,  traversing  all  the  subdivisions 
of  the  vesic.  'hat  of  the  second,  increasing  but  little  in 
height  and  breadth,  constitutes  the  Her;  while  that  of  the 
first  vesicle  is  continued  into  tlie  cerebral  hemispheres  to 
form  the  lateral  ventricles,  the  remainder  of  it  constituting 
the  third  ventricle,  which  includes  the  cavity  of  the  median 
portion  of  the  telencephalon  as  well  as  the  entire  cavity  of 
the  diencephalon. 


it 


I. 


f^BT 


4o6 


THK    nEVELOPMENT    OF   THE    HUMAN    UODV. 


During  the  differentiation  of  the  various  divisions  of 
tiie  brain  certain  flexures  appear  in  the  roof  and  floor,  and 
to  a  certain  extent  correspond  with  those  already  de- 
scribed as  occurring  in  the  embryo.  The  first  of  these 
flexures  to  appear  occurs  in  tlie  region  of  the  mid-brain, 
the  first  vesicle  being  bent  ventrally  until  it  comes  to  lie 
at  practically  a  right  angle  with  the  axis  of  the  mid-brain. 
This  mav  be  termed  the  mid-lmiin  flexure  (Fig.  216,  mj) 


Fir,  217   -Mkdian  I/)n-.;itui.inau  Skction  ok  thk  Brain  ok  an  Kmbkyo 
OK  THK  Thiko  Month.— (///9.) 

and  corresponds  with  the  head-bend  of  the  embryo.  The 
second  flexure  occurs  in  the  region  of  the  medulla  oblon- 
gata and  is  known  as  the  neck  flexure  (Fig.  216,  nf);  it 
correspond-  with  the  similarly  named  bend  of  the  embryo 
and  is  produced  by  a  bending  ventrally  of  the  entire  head, 
so  that  the  axis  of  the  mid-brain  comes  to  lie  almost  at 
right  angles  with  that  of  the  medulla  and  that  of  the  first 
vesicle  parallel  with  it.  Finally,  a  third  flexure  occurs  in 
the  region  of  the  metencephalon  and  is  entirely  peculiar 


TUF.   MVKLKNCKl'HAI.DX. 


407 


to  the  nervous  system;  it  consists  of  u  hendinK  ventrally 
of  the  floor  of  the  hind  brain,  the  roof  of  this  portion  of  the 
brain  not  being  affected  by  it,  and  it  may  consequently 
be  known  as  the  pous  firxure. 

In  the  later  development  the  pons  flexure  practically 
disappears,  owing  to  tlie  development  in  this  region  of  the 
transverse  fibers  and  nuclei  of  the  pons,  but  the  mid- 
brain an-'  leck  flexures  persist,  though  greatly  reduced  in 
acuteness,  the  axis  of  the  anterior  portion  of  the  adult 
brain  being  inclinH  to  that  of  the  medulla  at  an  angle  of 
about  i.u  degrees. 

The  Development  0}  the  Myelcncephnlon.—lw  its  poste- 
rior  portioi;    tlie   myelencephalon   closely  resembles  the 
spinal  cord  and  has  a  very  similar  development.     More 
anteriorly,  however,  the  roof-plate  (Fig.  218,  rp)  widens 
to  form    an  exceedingly  thin   membrane,   the  posterior 
velum;  with  the  broadening  of  the  roof-plate  tliere  is  as- 
sociated a  broadening  of  the  dorsal  portion  of  the  brain 
cavity,  the  dorsal  and  ventral  zones  bending  outward, 
until,  in  the  anterior  portion  of  the  after-brain,  the  mar- 
gins of  the  dorsal  zone  have  a  lateral  position,  and  are,  in- 
deed, bent  ventrally  to  form  a  reflected  lip  (Fig.  218).    The 
portion  of  the  fourth  ventricle  contained  in  this  division 
of  the  brain  Ijecomes  thus  converted  m'lo  a  broad  shallow 
cavity,  whose  floor  is  formed  by  the  ventral  zones  sepa- 
rated in  the  median  line  by  a  deep  groove,  the  floor  of 
which  is  the  somewhat  thickened  floor-plate.     About  the 
fourth  month  there  appears  in  the  roof-plate  a  transverse 
groove  into  which  the  surrounding  mesenchyme  dips,  and, 
as  the  groove  deepens  in  later  stages,  the  mesenchyme  con- 
tinued within  it  becomes  converted   into  bl-^d- vessels, 
forming   the  rhorioid  plexus  of    the   fourth  ventricle,  a 
structure  wliich,  as  may  be  seen  from  its  development, 
does  not  lie  within  the  cavity  of  the  ventricle,  but  is 


i    '■' 


»    M 


408 


rm:  ni.vKr.oi'MKNT  ok  the  hlman  uodv. 


separated  from  it  l)y  the  portion  of  the  roof-plate  winch 
forms  the  lloor  of  the  groove. 

In  embryos  of  about  9  ">"'  the  difTerentiation  of  the 
dorsal  and  ventral  zones  into  ependymal  and  mantle 
layers  is  clearly  visible  (V\g.  218),  and  in  the  ventral  zone 
the  marginal  velum  is  also  well  developed.  Where  the 
fibers  from  the  sensorv  ganglion  of  the  vagus  nerve  enter 
the  dorsal  zone  an  oval  area  (l<ig.  218,  fs)  is  to  l)e  seen 
which  is  evidently  comparable  to  the  oval  bundle  of  the 


Vic.   21.S.-TK.\Nsvt;Hsiv  Section-   tiikoit.ii   tiii-    Mkdilla  ()bi.<)N«'-\ta 

OI-    AN     IvMBRVO    or    '>.!     MM. 

,/'    Dorsal   /cne;  ;/',  n.-nr-platc;  js,  fasciculus  solitarius;  /,  lip;   '^^„''""/- 
I)latc;  re,  ventral  /one;  y  and  A//,  tenth  and  twclttli  nerves.-    (//(>.) 

cord  and  consequently  with  the  column  of  Burdach.  It 
gives  rise  to  the  solitary  fasciculus  of  adult  anatomy,  and 
in  embrvos  of  1 1  to  13  mm.  it  becomes  covered  in  by  the 
fusion  of  the  reflected  lip  of  the  dorsal  zone  with  the  sides 
of  the  myelencephalon,  this  fusion,  at  the  same  time, 
drawing  the  margins  of  the  roof-plate  ventrally  to  form  a 
secondary  lip  ( Tig.  219).  Soon  after  this  a  remarkable  mi- 
gration ventrally  of  neuroblasts  of  the  dorsal  zone  begins. 
Increasing  rapidly  in  lunnber  the  migrating  cells  pass  on 


THE    MVKI.KNCKrHALON. 


409 


either  side  of  the  solitary  fasciculus  toward  the  territory  of 
the  vritral  zone,  and,  passinij  vetitrally  to  the  vential 
portion  of  the  mantle  layer,  into  which  fibers  have  pene- 
trated and  which  becomes  the  jornuitw  ntuularis  (Ki^. 
219,  fr),  thev  (lifTcrentiate  to  form  the  olivaty  hotly  iol). 

The  thickening  of  the  tloor  plate  K'iv^'^  opportunity  for 
fibers  to  pass  across  the  median  line  from  one  side  to  the 
other,  and  this  opportunity  is  taken  advantajje  of  at  an 
early  staj;e  bv  the  axis-cylinders  of  the  neuroblasts  of  the 


iMC.    21'). 


(ir 


JOT 


-Tkansvkkse  Skction   TiiK<)r<;n  the   Medilua   ()hm)N<;ata 

OK  AN    IvMBRYO  OF    \HorT    lUt.IlT  WEEKS. 

ilivary 

OSSill 


AscenditiK  n.ol  ..f  the  triKetninus;  /,,  reticular  f<.rmati..n;  o/,  divar 
'    body;    s/,  solitary   fasciculus;   Ir,  restifurni  l...(iy;    .\//,  l.yiH.Kl.-ssi 


nerve.     (His.) 


ventral  zone,  and  later,  on  the  establishment  of  the  olivary 
bodies,  otl-  -  fibers,  descending  from  the  cerebellum,  de- 
cussate in  this  region  to  pass  to  the  olivary  body  of  the 
opposite  side.  In  the  lower  part  of  the  medulla  fibers 
from  the  neuroblasts  of  the  nuclei  of  Goll  and  Burdacli. 
which  seem  to  be  developments  from  the  mantle  layer  of 
the  dorsal  zone,  also  decussate  in  the  substance  of  the 
floor-plate;  these  fibers,  known  as  the  arcuate  fihrrs,  pass  in 
part  to  the  cerebellum,  associating  themselves  with  fibers 
ascending  from  the  spinal  cord  and  with  the  olivary  fibers 


^lEsrr: 


■WW 


!  I 


410  riiK  i>kvi'.ioi'Mi;nt  (•!•  tiik  iiiman   iiodv. 

to  form  a  round  huudU'  sitiiaUd  in  the  dorsal  portion  of 
the  marginal  vtluni  and  known  as  the  nsiijorm  body  (Fij;. 
219.  tr). 

'\'\w  principal  (lilTcn-ntiations  of  tlu-  zunis  of  tlu-  niyiKii 
liplialnu  may  Ik-  stated  in  tabular  fortn  as  follows: 

UiMif  pliti' posii-rinr  vi'luiii. 

/  N'lirki  of  tiriiiitiiitinii  of  sinsory  rt><ii>   i>l 

\      cranial  nirvts. 

Dors.  1 1 /cms,     5  Niiiki  nf  (",i>ll  anil  Miinladi. 

\  Tlic  iilivary  ln><liis. 

{Nuclei  cf  <»ri>;in  of  iIh-  motor  roi.is  o,"  ,1  i;<ial 
nirvts. 
Till'  ri-tiiular  formation. 
I'loor  |ilali- Tlif  mi-ilian  raplii'. 

riu  Dciclopmctit  0}  the  Mdcnciphtihm  mid  Isthmus.  - 
Our  knovvlfdse  of  the  devdopnient  of  the  tnetencei)halon. 
isthmus,  and  mesencephalon  is  by  no  means  as  complete 
as  is  that  of  the  myelencephalon.  The  pons  develops  as  a 
thickeniuij  of  the  portion  of  the  brain  floor  which  forms 
the  anterior  wall  of  the  pons  flexure,  and  its  transverse 
fd)ers  are  well  developed  by  the  fourth  month  (Mihal- 
kovicz),  but  all  details  rejrardiuK  the  ori,i,dn  of  the  pons 
nuclei  are  as  yet  wantinjj.  If  one  may  arsrue  from  what 
occurs  in  the  myelencephalon,  it  seems  probable  that  the 
reticular  formation  of  the  metencephalon  is  derived  from 
the  ventral  zone,  and  that  the  median  raphe  represents 
the  floor-plate.  Furthermore,  the  relations  of  the  pons 
nuclei  to  the  reticular  formation  on  the  one  hand,  and  its 
connection  by  means  of  the  transverse  pons  fibers  with 
the  cerebellum  on  the  other,  suggest  the  possibility  that 
they  may  be  the  metencephalic  representatives  of  the 
olivary  bodies  and  be  formed  by  a  migration  ventrally  of 
neuroblasts  from  the  dorsal  zones. 

The  cerebellum  is  formed  from  the  dorsal  zones  and 
roof-i)late  of  the  metencephalon  and  is  a  thickening  of  the 


TJir.  Mrn-.M'-.i'iiAioN. 


411 


tissue  iiiunediatcly  autirtor  to  the  front  vi\^v  of  the  poste- 
rior velum.     This  latter  structure  has  in  early  sta^'es  a 
rhoi.ihoidal  shape  (Imk'.  220,  A)  which  causes  the  cere 
bellar  thickenitiK  to  appear  at  first  as  if  composed  of  tw<. 
lateral  portions  inclined  ol)li(luely  toward  one  another. 
In    reality,    however,    the    thickening   extends    entirely 
across  the  roof  of  the  brain  (l'ii(.  220,  H).  the  roof-plate 
probably  beinii  invaded  by  cells  from  the  dorsal  /.ones 
and  so  jr'ivini,'  rise  to  the  vnmis,  while  the  lobes  are  formed 
directly  from  the  dorsal  /.ones.     During'  the  second  month 
a  j;roove  appear-  on  the  ventral  surface  of  each  lobe, 


U AHiii'i    1';mhk\'i  oi-   1(> 
\i.r  I'Imhk' 
r,  CiTi'hi'lliuii;  )>i,  mid-brain.      (  M ili.ilKoiii-z.) 


!.'„;      ■>■>()  \      DoKSAI,   VlICW   OI-    Till-     liKAI.V   (IK    A 

MM.;"/.',  M.:i.i.\N  I.()N..iTri.rNAU  SiXTioN  ..I-  aCai.i-  I'.MHKVOor  .<  CM 


marking  out  an  area  which  becomes  the  jlocculus,  and 
later,  durini;  the  third  month,  transverse  furrows  appear 
upon  the  vermis  dividino;  it  into  five  lobes,  and  later  still 
extend  out  upon  the  lobes  and  increase  in  number  to  pro 
duce  the  lamellate  structure  characteristic  t)f  the  cere 

bellum. 

Tiie  histoscnetic  development  of  the  ccrel)ellum  at  iirst 
proceeds  aloni,'  the  lines  which  have  already  been  de- 
scribed as  typical,  but  after  th.e  development  of  tin  man 
tie  layer  the  cells  liniui;  the  greater  portion  of  the  cavity 
of  the  ventricle  cease  to  multiply,  only  those  which  are 


ii»i ,  ■ 


f'^-i 


■:M 


i  r 


m 


TIIK    DKVKLOrMENT    OK    TIIP:    HUMAN    liODY. 


i     i 


I    I 


situated  in  the  roof-plate  of  the  nieteneephalon  and  along 
the  line  of  junction  of  the  cerebellar  thickening  with  the 
roof-plate  continuing  to  divide.  The  indifferent  cells 
formed  in  these  regions  migrate  outward  from  the  median 
line  and  forward  in  the  marginal  velum  to  form  a  super- 
ficial laver,  known  as  the  epithelioid  layer,  and  cover  the 
entire  surface  of  the  cerebellum.     The  cells  of  this  layer, 

like  those  of  the  man- 
tle, dilTerentiate  into 
neuroglia  cells  and 
neuroblasts,  the  latter 
for  the  most  part  mi- 
grating centrally  at  a 
later  stage  to  mingle 
with  the  cells  of  the 
mantle  layer  and  to 
become      transformed 

into  the  qninular  cells 
I'K,.  221.     Di.\i;k.\m  Rkpresentim;  thiv  .     .  i     u 

Dii  TEKENTiATioN  (H  THE  Cekebeular     "I   tlic  cereoeiiar  cor- 
<-KLLs.  t^^x.       The     neuroglia 

TliL-  circles,  indilYert'iit  cells;  circles  witli  ,,                 •        +  +i       ^..^ 

(lots,  nenn-Klia  cells;   sliaded  cells,  Rer-  CCllS  remam  at  tnc  SUr- 

ininal  cells;  circles  witli  crnss,  germinal  f,^^,^.    however   forming 

cells  in  initdsis;  hlack  cells,  nerve-cells.  '                       ' 

/.,    Lateral  recess;    .)/,    nudian   fiirn>\v,  the  principal  COnstltU- 

iind   A',  lldorof  /l,  fourth   ventricle.    -  „,    ^r  ,i  ,^  ,,.,*^r  ,^,.    -,<. 

(Siiiahrr  )  *-'"^   ^^    '-'^*^   OUtCr  Or,   JS 

it  is  now  termed,  the 
molecular  layer  of  the  cortex,  and  into  this  the  dendrites  of 
the  Purkinje  cells,  probably  derived  from  the  mantle  layer, 
project.  The  migration  of  the  neuroblasts  of  the  epithe- 
lial layer  is  probably  completed  before  birth,  at  which  time 
but  few  remain  in  the  molecular  layer  to  form  the  stellate 
cells  of  the  adult.  The  origin  of  the  dentate  and  other  nu- 
clei of  the  cerebellum  is  at  present  unknown,  but  it  seems 
])robab!e  that  they  arise  from  cells  of  the  mantle  layer. 
The  nerve-fibers  which  form  the  medullary  substance  of 


the  cerebellum  do  not  make  tlieir  appearance  ut"il 
about  the  sixth  month,  when  they  are  to  be  found  in  the 
ependymal  tissue  on  the  inner  surface  of  the  layer  of  gran- 
ular cells.  Those  which  are  not  commissural  or  associa- 
tive in  function  converj^e  to  tlie  line  of  junction  of  the 
cerebellum  with  the  pons,  and  there  pass  into  the  mar- 
ginal velum  of  the  pons,  myelencephalon,  or  isthmus  as 
the  case  may  be. 

The  dorsal  surface  of  the  isthmus  is  at  first  barely  dis- 
tinguishable from  the  cerebellum,  but  as  development  pro- 
ceeds its  roof-plate  undergoes  changes  similar  to  those 
occurring  in  the  medulla  oblongata  and  becomes  converted 
into  the  anterior  velum  and  lakc  of  Vieussens.     In  the 
dorsal  portion  of  its  marginal  velum  fillers  passing  to  and 
from  the  cerebellum  appear  and  form  the  superior  pedun- 
cle of  the  cerebellum  (brachiurn  coniunctivum),  while  ven- 
trally  fibers,  descending  from  the  more  anterior  jjortions 
of  the  brain,  form  the  crura  cerebri.     Nothing  is  at  present 
known  as  to  the  history  of  the  gray  matter  of  this  division 
of  the  brain,  although  it  may  be  presumed  that  its  ventral 
zones  take  part  in  the  formation  of  the  tegmentum,  while 
from  its  dorsal  zones  the  nuclei  of  the  brachia  conjunctiva 
are  possibly  derived. 

The  following  tabic  gives  the  origin  of  the  principal  structures 
of  the  metencephalon  and  isthnnis: 

f  I'oslcrior  velum.  Auturior  vcliiiii. 

Rnof-platc,    ^  Vermis  of  cerehelliim.  Valve  of  \ieiisse!is. 

/    Kdhes  of  cerehelliim.  Hraeliia  eoiiiimetiva 

I    IHoeculi. 

Dorsal  zones 1   Nuclei    of    tcrinination    of 

\       sensory  roots  of  cranial 

I        nerves. 

\   Pons  nuclei. 

r  Nuclei  of  ori^'in  of  motor     Posterior  part  of  crura 

Ventral  zones J       roots  of  cranial  nerves.  cerebri. 

\  Reticular   formation.  Posterior  part  of  tc^- 

1  mentum. 

Floor-plate, Median  raphe.  Median  raphe. 


W* 


I 
la 


M 


i  i 


4.14  THE    DF.VELOPMENT    OF    THE    HUMAN    HOOV. 

The  Development  of  the  Mesencephalon.— Our  knowledge 
of  the  development  of  this  portion  of  the  brain  is  again 
very  imperfect.  During  the  stages  when  the  flexures  of 
the  brain  are  well  marked  (Figs.  216  and  217)  it  forms  a 
very  prominent  structure  and  possesses  for  a  time  a  capa- 
cious cavity.  Later,  however,  it  increases  in  size  less 
rapidly  than  adjacent  parts  and  its  walls  thicken,  the  roof- 
and  floor-plates  as  well  as  the  zones,  and,  as  a  result,  the 
cavity  becomes  the  relati^  e  y  smaller  canal-like  iter.  In 
the  marginal  velum  of  it'  'antral  /one  fibers  appear  at 
about  the  third  month,  fornung  the  anterior  portion  of  the 
crura  cerebri,  and,  at  the  same  time,  a  median  longitudi- 
nal furrow  appears  upon  the  dorsal  surface,  dividing  it  into 
two  lateral  elevations  which,  in  the  fifth  month,  are  di- 
vided transversely  by  a  second  furrow  and  are  thus  con- 
verted from  corpora  bigemina  (in  which  form  they  arc 
found  in  the  lower  vertebrates)  into  corpora  quadrigemina. 

Xothing  is  known  as  to  the  ditTerentiation  of  the  gray  matter 
of  the  dorsal  and  ventral  zones  of  the  mid  brain.  l-Voni  the 
relation  of  the  parts  in  the  adult  it  seems  probable  that  in  addi 
lion  to  Ihc  nuclei  of  origin  of  the  oculomotor  and  trochlear 
njrves,  the  ventral  zones  give  origin  to  the  gray  matter  of  the 
tegmentum,  which  is  the  forward  continuation  of  the  reticular 
formation.  vSiniilarly  it  may  he  supposed  that  the  corpora  C|uad- 
rigemina  arc  developments  of  the  dorsal  zones,  as  may  also  be 
the  red  niichi,  whose  relations  to  the  superior  peduncles  of  the 
cerebellum  suggest  a  comparison  with  the  olivary  bodies  and 
the  nuclei  of  the  pons. 

A  tentative  scheme  representing  the  origin  of  the  mid-brain 
structures  may  be  stated  thus: 

R(M)f-i)l:ito,    C^) 

(  Corpora  qiuulrijreiiiina. 
Dorsal  zones (  Red  nuclei. 

,  Xuclci  of   origin  of    tlie    third   ;iud 

I       fourth  nerves. 
Ventral  zones I  Anterior  part  of  tegmentuiu. 

I  Anterior  part  of  crura  cerebri. 
Floor  plate, Median  raphe. 


fTi. 


IIIK    niFNCKrilAI ON. 


415 


The  Development  0}  the  Dienceplnilon.—X  transverse 
section  tlirouK'h  the  diencephalon  of  an  embryo  of  about 
five  weeks  (Fiff-  222)  shows  clearly  the  (HtTen-ntuition  of 
this  portion  of  the  brain  into  the  typical  zones,  the  roof- 
plate  {rp)  being  represented  by  a  thin- walled,  sonu  what 
folded  area,  the  floor-plate  ijp)  by  the  tissue  forming  the 
floor  of  a  well-marked  ventral  groove,  while  each  lateral 
wall  is  divided  into  a  dorsal  and  ventral  zone  by  a  groove 
known  as  the  sulcus  Monroi 
(Sm),  which  extends  forward 
and  ventrally  to^-ard  the 
point  of   origin  •  ^  optic 

evagination  (Fig.  ^).  At 
the  posterior  end  of  the  ridge- 
like elevation  which  repre- 
sents the  roof-plate  is  a 
rounded  elevation  (Fig.  223, 
p)  which,  in  later  stages, 
elongates  until  it  almost 
reaches  the  dermis,  forming 
a  hollow  evagination  of  the 
brain  roof  known  as  the 
pineal  process.  The  distal 
extremity  of  this  process  en- 
larges to  a  sac-like  structure 
which   later    becomes    lobed, 

and,  by  an  active  proliferation  of  the  cells  lining  the  cavi- 
ties of  the  various  lobe.,,  finally  become.,  a  solid  structure, 
the  pineal  body.  The  more  proximal  portion  of  the  evag- 
ination, remaining  hollow,  forms  the  pineal  stalk,  and  the 
entire  structure,  body  and  stalk,  constitutes  what  is 
known  as  the  epiphysis. 

The  significance  of  this  organ  in  the  Mammalia  is  doubtful. 
In  the  Replilia  and  other  lower  forms  the  outgrowth  is  douhlc, 


l'"iii.  222.-  Tk.\.\s\i;ksiv  Si:c- 
TioN  <>i'  TiiK  TirAi,.\Mi:\ci:rii- 

AI.o.N  OK  .\\   IvMUKVO  UK   I'"l\IC 
W'lvlvKS. 

1/:,  Dorsal /.one;  //>,  floor-pl.iti- ; 
//>,  roof-])late;  S»i,  sulcus 
Monroi;  rz,  ventral  zone.-— 
(Ills.) 


f 


\m 


m 


4i6 


THK    nEVEI-OPMKNT    OF    THE    HUMAN    IIOPY. 


f 


'     i 


a  secondary  (.utgnnvth  arising  from  the  base  or  from  the 
anterior  wall  of  the  primary  o,ie.  This  anterior  evaKMnat.on 
elon«ates  until  it  reaches  the  dorsal  epidermis  of  the  head  ami, 
here  expanding.  <levelops  into  an  unpaired  eye.  the  epidermis 
which  overlies  it  becoming  converted  into  a  transparent  cornea. 
In  the  Mamnali  I  this  anterior  process  does  not  develop  and  the 
epiphysis  in  1'  .>c  forms  is  comparable  only  to  the  posterior  pro- 
cess ./."  the  .vcptilia. 

In  addition  to  the  epiphysial  evaginat ions   another  evagina 
tion  arises  from  the  roof  plate  of  the  first  brani  vesicle,  further 
forward,  in  the  region  which  becomes  the  median  P;'--  '<>;^;  J  "■ 
telencephalon.     This  paraM>ysis.  as  it  has  been  called,  h as  be.  n 
„bserved  in  the  lower  vertebrates  and  ^^^"^^  ^^""''"T^^^^^ 
lenka),  but  up  to  the  present  has  not  been  f.mnd  in  other  gnmps 
„f  the  Mammalia.     It  seems  to  be  comparable  to  a  choriou 
plexus  which  is  evaginated  from  the  brain  surface  instead  of 
bdng  invaginated  a"  is  usually  the  case.     There  is  no  evidence 
that  a  paraphysis  is  developed  in  the  human  brain, 

The  portion  of  the  roof-plate  which  lies  in  front  of  the 
epiphysis  represents  the  yeluni  interpositum  of  the  adult 
braini  and  it  forms  at  first  a  distinct  ridge  (Fig.  223).  At 
an  early  stage,  howeyer,  it  becomes  reduced  to  a  thin 
membrane  upon  the  surf.xe  of  which  blood-yesseis,  de- 
yeloping  in  the  surrounding  mesenchyme,  arrange  them- 
selves at  about  the  third  month  in  two  longitudmal 
plexuses,  which,  with  the  subjacent  portions  of  the  velum, 
become  invaginated  into  the  cavity  of  the  third  ventricle 
to  form  its  chorioid  plexus. 

The  dorsal  zones  thicken  in  their  more  dorsal  •  nd  ante- 
rior portions  to  form  massive  structures,  the  optic  thalamt 
(Figs  217,  V2,  and  223,  ot),  which,  encroaching  upon  the 
cavity  of  the  ventricle,  transform  it  into  a  narrow  slit-like 
space,  so  narrow,  indeed,  that  at  about  the  hfth  month 
the  inner  surfaces  of  the  two  thalami  come  m  contact  in 
the  median  line,  forming  what  is  known  as  the  middle  or 
son  commissure.  More  ventrally  and  posteriorly  another 
thickening  of  the  dorsal  zones  occurs,  giving  rise  on  each 


TllK    DIKNCKPHALON. 


417 


side  to  the  puhinar  of  the  thalamus  and  to  an  external 
geniculate  body,  and  two  ridges  extending  backward  and 
dc-.ally  from  the  latter 
structures  to  the  thicken- 
ings in  the  roof  of  the  mid- 
brain which  represent  the 
anterior  corpora  quadri- 
gemina,  give  a  path  along 
which  the  nerve-fibers 
which  constitute  the  ante- 
rior hrachia  pass. 

From  the  ventral  /ones 
what  is  known  as  the  sub- 
thalamic region  develops,  a 
mass    of    fibers    and    cells 
whose  relations  and  devel- 
opment are  not  yet  clearly 
understood,  but  which  may 
be  regarded  as  the  forward 
continuation    of    the    teg- 
mentum and  reticular  for- 
mation.   In  the  median  line 
of  the  floor  of  the  ventricle 
an  unpaired  thickening  ap- 
pears, representing  the  cor- 
pora albicantia,  which  dur- 
ing   the    third    month    be- 
comes divided  bv  a  median 
furrow   into    two    rounded 
eminences;      but     whether 
these    structures    and    the 
posterior  portion  of  the  tuber  cinercum,  which  also  de- 
velops from  this  region  of  the  brain,  are  derivatives  of 
the  ventral  zones  or  of  the  floor-plate  is  as  yet  uncertain. 
35 


Vu..  22,^.-  Ddksai.  View  of  the 
Hkain,  the  Rook  ok  the  Lat- 
eral Ventricles  being  Re- 
moved, OK  AN  KmBRVO  ok  13.6 
MM. 

/),  Anterior  hrachiuin ;  fi,',  external 
geniculate  body;  rf,  cliorioid 
plexus;  cqa,  anterior  corpus 
quadrigeininum ;  //,  hippocam- 
pus; /(/,  liippocanipal  I'lssure;  <>/, 
optic  tlialanuis;  />.  pineal  body; 
r/>,  roof-plate. — {His.) 


^ 


'  1 


U 


iW 


!  i 

n 


u  i 


4,8  THK    DF.VELOPMENT    OF     THE    HUMAN    HOnY. 

Assumitig  that  the  albicanlia  and  ^'l^  ^"'^  ,^^^^J^7,„",[: 
tlerivcd  from  the  ventral  zones,  the  oriRms  <'V'\'  ,^^W^  .« 
fonued  from  the  walls  ..f  the  diencephalon  may  be  tabulated  as 

follows : 

,  Vcliiiii  imiTposituiii 

K.H.f-platf -^  Hi)il)liysis. 

r Optic  thalami. 

.)ur^aWum.s I'ulvinarcs. 

I  ICxleriial  j^oiiKiilatc  Inxlifs. 

/•Suhthalaiiiic   region. 

,       „  '  Corpnra  alhicaiitia. 

ITuIkt  ciiu'reuin  (in  part). 

'risstif  of  iiiid-vcnlral  line. 
I'lodr-platc, 

The  Dexdopmcnt  oj  the  rdcnaphalon.-Vor  cotivcnieiice 
of  description  the  telencephalon  may  be  regarded  as  con- 
sistin.r  of  a  median  portion,  which  contains  the  anterior 
part  of  the  third  ventricle,  and  two  lateral  outgrowths 
which   constitute   the   cerebral    hemispheres.     Ihe    roof 
of  the  median  portion  undergoes  the  same  transformation 
as  does  the  greater  portion  of  that  of  the  diencephalon 
and  is  converted  into  the  anterior  part  of  the  velum  inter- 
positum  (Fig.  224.  vi),  wbich  anteriorly  passes  into  the 
anterior  wall  of  the  third  ventricle,  the  lamina  tcrminahs 
(It),  a  structure  which  is  to  be  regarded  as  formed  by  the 
union  of  the  dorsal  /.ones  of  opposite  sides,  since  it  lies 
■ntirely  dorsal  to  the  anterior  end  ol  the  sulcus  Monroi. 
From  the  ventral  part  of  the  dorsal  zones  the  optic  evagi- 
nations  are  formed,  a  depression,  the  optic  recess   (or), 
marking  their  point  of  origin. 

The  ventral  zones  are  but  feebly  developed,  and  form 
the  anterior  part  of  the  subthalamic  region,  while  at  the 
anterior  extremitv  of  the  floor-plate  an  evagination  oc- 
curs the  infumlibulay  recess  (ir),  which  elongates  to  form 
a  funnel-shaped  structure  known  as  the  hypophysis.  At 
its  evtremitv  the  hvpophysis  comes  in  contact  during  the 
r.fth  week  w'itl.  the  enlarged  extremity  of  Rathke's  pouch 


TllK    TKI.l'.N^KI'HALON. 


419 


formed  by  an  invagination  of  the  roof  of  tlu-  oral  sinus 
(sec  p.  3CX)),  and  applies  itst-lf  closely  to  the  posterior  sur- 
face of  this  (Fig.  2\-])  to  form  with  it  the  pituitary  body. 
The  anterior  lobe  at  an  early  stage  separates  from  the 
mucous  membrane  of  the  oral  sinus,  the  stalk  by  which 
it  was  attached  completely  disappearing,  and  toward  the 
end  of  the  second  month  it  begins  to  send  out  processes 
from  its  walls  into  the  surrounding  mesenchyme  and  so 


nr  rr 

Vu:.  224. -Median  I.oxi.itiiunau  Skctiox  oi-  the  Brain  ok  an    Ivm- 

BRVI)  OK    1.^.6   MM. 

br.  Anterior  l)racliiiiiii ;  rt;,  corjjiis  Kcniciilatuin  extermini ;  cs,  corpus  slria- 
tuni;  /;,  cerebral  licinisplierc ;  -r,  infun(lil)ular  recess;  //,  lamina  terni- 
inalis;  or,  optic  recess;  ot,  o])tic  tlialanius;  ^,  pineal  process;  sni,  sul- 
cus Monrlii;  st,  suhllialaniic  region;  ri,  velum  interi)ositum.— (///v.l 


ill 


becomes  converted  into  a  mass  of  solid  epithelial  cords 
embedded  in  a  mesenchyme  rich  in  blood  and  lymphatic 
vessels.  The  cords  later  on  divide  transversely  to  a 
greater  or  less  extent  to  form  alveoli,  the  entire  structure 
coming  to  resemble  somewhat  the  parathyreoid  bodies 
(see  p.  314),  and,  like  these,  having  the  function  of  pro- 
ducing an  internal  secretion.  The  posterior  lobe,  de- 
rived from  the  brain,  retains  its  connection  with  that 


t^l 


i  1 


n 


;i 


I 


tU 

I 


M  t 


420  -I. IK    I.KVKIOPMKNT    OF    THK    HUMAN    HODY. 

structure,  its  stalk  being  the  iufuudihulum  but  its  ter- 
nunal  no  tion  does  not  undergo  such  extensive  modihca- 
Tns  as  does  the  anterior  lobe,  although  it  is  claimed  tluU 
it  gives  rise  to  a  glandular  epithelium  which  may  become 
arranged  so  as  to  form  alveoli.  ,     ,  ^      1 

The  cerebral  hemispheres  are  formed  from  the  lateral 
portions  of  the  dorsal  zones,  each  possessing  also  a  pro- 
ongation  of  the  roof-plate.     From  the  more  ventral  por^ 
tion  of  eaduiar^^il  ^-«"^'  ^here  is  formed  a  thickemng,  the 
ZprJ^^a-^^-  -M.  -.  --^  -7.  VI  .).  a  structure 
wlich  is  for  the  telencephalon  what  the  optic  thalamus 
is  for  the  diencephalor,  while  from  the  more  dorsa   por- 
tion there  is  formed  the  remaining  or  mantle  (/.a//u//)  por- 
tions of  the  hemispheres  (Figs.  224,  h,  and  217,  VI  4)- 
When  first  formed,  the  Ik.     spheres  are  slight  evagina- 
tions  from  the  median  portion  of  the  telencephalon,    h 
openings  bv  which  their  cavities  communicate  with  the 
third  ^x>ntricle.  the  foravnna  of  Mon>o,  bemg  relatively 
verv  large  (Fig.  .24).  but,  in  later  stages  a^g.  217).  they 
increase  more  markedly  and  eventually  surpass  all  the 
other  portions  of  the  brain  in  magnitude,  overlapping  and 
completelv  concealing  the  roof  and  sides  of  the  dienceph- 
alon  and 'mesencephalon   and   also  the  anterior  surface 
of  the  cerebellum.      In  this  enlargement,  however    the 
foramina  of  Monro  share  only  to  a  slight  extent  and  con- 
secmentlv  become  relatively  smaller  (Fig.  217),  forming 
in  the  adult  merely  slit-like  openings  lying  between  the 
lamina  terminalis  and  the  optic  thalami  and  having  for 
their  roof  the  anterior  portion  of  the  velum  interpositum. 
The  velum  interpositum,-that  is  to  say,  the  roof-plate, 
-where  it  forms  the  roof  of  the  foramen  of  Monro,  is 
prolonged  out  upon  the  dorsal  surface  of  each  hemisphere, 
and   becoming  invaginated,  forms  upon  it  a  groove.     As 
the  hemispheres,  increasing  in  height,  develop  a  mesial 


THE    IKLENCEPHALON. 


42 


^V 


wall  the  groove,  wliicli  is  the  so-called  chorioiiUil  fissure, 
comes  to  lie  along  the  ventral  edge  of  this  wall,  and  as  the 
.rrowth  of  the  hemispheres  continues  it  becomes  more  and 
more  elongated,  being  carried  at  first  backward  (l-ig.  225). 
then  ventrallv,  and  finallv  forward  to  end  at  the  tq)  of 
the  temporal  lobe.    After  the  establishment  of  tlie  grooves 
the  mesenchvme  in  their  vicinity  dips  into  them,  and, 
developing  b'lood-vessels,  becomes  the  clumoid  plexuses 
of  the  lateral  ventricles,  and  at  tirst  these  plexuses  grow 
nmch    more    rapidly    than    the 
ventricles,  and  so  till  them   al- 
most completely.     Later,   how- 
ever,   the    walls    of    the    hemi- 
spheres gain  the  ascendancy  in 
rapidity    of    growth     and     the 
.   plexuses  become  relatively  much 
smaller.     vSince   the  portions  of 

the  roof-plate  which   form   the 

chorioidal  fissures   are  continu- 
ous with  the  velum  interpositum 

in  the  roofs  of  the  foramina  of 

Monro,  the  chorioid  plexuses  of 

the  lateral  and  third  ventricles 

become  continuous  also  at  that 

point. 

The  mode  of  growth  of  the  chorioid  fissures  see.ns  to 
indicate  the  mode  of  growth  of  the  hemispheres.  At 
first  the  growth  is  more  or  less  equal  in  all  directions,  but 
later  it  becomes  more  extensive  posteriorly,  there  being 
irore  room  for  expansion  in  that  direction,  and  when 
further  extension  backward  becomes  difficult  the  posterior 
extremities  of  the  liemispheres  bend  ventrally  toward  the 
base  of  the  cranium,  and,  reaching  this,  turn  forward  to 
form  the  temporal  lobes.      As  a  result  the  cavities  of  the 


Fig.  225.— Meuian  I.onoi- 
TuuiNAU  Section  01'  the 
Brain  of  an  Kmbkyo 
Calf  of  .S  cm. 

cb,  CerelieHum ;  cp,  chorioid 
plexus;  0,  corims  strui- 
luni;  /.\/,  foramen  of  Mon- 
ro; i>i,  hypopliysis;  m,  mid- 
brain; oc,  optic  aminiis- 
surc;  /,  posterior  part  of 
the  thalamcnce])halon. — 
(Milhilkovicz.) 


'Ul 


§j! 


ill 


■  ''*"'T^--'^?^^ 


i 

■    ! 

i 

422  TIIK    DEVELOPMENT    OK    THE    IILMAN    hOUV. 

■     1  ii,f>  intt-ril  ventricles,  in  addition   to  beinK 

;;^i:::;d  l^ntrally  to  fonn  the  lateral  or  deseen^m. 
orn  md  the  eorpus  striatum  likewise  extends  backward 
^^L'UP  c'lach  len,poral  lobe  as  a  slender  pn.c^s    no.v. 
as  the  tail  of  the  caudate  t.ueleus.     In  ^^*»f  ^^^  '  ^ 

anterior  and  lateral  horns,  the  ventncles  of  tl^J  - 
brnin  also  possess  T)osHr  horns  exteiuhng  backward 
i  no  the  cJipital  portions  of  the  henhspheres  these  por- 
t^ns  on  account  of  the  K-ater  persistence  of  the  nnd- 
;rin  ilexure  (see  p.  406).  being  enabled  to  de^•elop  to  a 
L-reate'-  extent  than  in  the  lower  nuunnials. 
'  n'e  schetne  of  the  origin  of  parts  in  the  telencephalon 
mavbe    i.ated  as  follows: 


Kdot-pUil' 


pors.il  zoiu's, 


Ml   MlslMlKKKS. 
MllJlAN    l-ART.  ■   ,     ,  r 

Antirior  i)irt  01  Ntitnu       ( 
ini.rpnsitiiiu- 


Ventral  zones, 


t 


( 


I.aniin  >  tcrminiUs. 
(  iptic  ev.i^inalions. 

Anterior    p  irt     of     sitli- 

tlialaiiiic  region. 
Anterior    i)irt    of    tnt>er 

ein-rviiiii. 


l^      sure. 

I'alliuni. 
I  Corpus  striatum. 
1  <  )lfactory  hull's   (.sec  p, 
(      4271. 


rl,c  ConvMons  oj  On-  ;/r„„V/./,...-,.^lhe  Krowll. 
UK.  hcnisphcn-s  t„  for,,,  .l.c  vo.ununous  ^<n.clur- '^^ 
i„  „H.  a,U,U  ,k.„c,„lH  .nainly  upon  an  --="--;^^^ 
,v,lliu.n  The-  corpus  striatum,  alllK.UKli  >t  takes  part  m 
U  e  onKatio,,  of  eacl,  he.nisplK.rc,  ucvcrthdess  tloes  „ot 
r  L'in  other  directions  as  rapidly  and  "tensue^-  - 
t„e  pailiun,,  and  '»"-,  eve,,  ,n  very  eariy  s  K  s  a^;.^^ 
pression  appears  upon  the   surtacc   01    inc  i 

^  1  :-cit.i-itcd  (lM«'  226).    This  depression  IS 

where  the  corpus  is  situated  11  i,,.  --^v- 


TIIK    CKREltRAL    CONVOLUTIONS. 


423 


the   fossa  Sylvii,  and  for  a  considerable   iKTiod  it  »s  l.ic 
only  siKMi  of  ineciualily  of  K^rowtl.  on  the  (iuter  surfaces  of 
the  hemispheres.     Upon  the  mesial  surfaces,  however,  at 
•ihout  the  time  that  the  chorioid  fissure  appears,  another 
linear  depression  is  formed  dorsal  to  the  chorioid,  and 
when  fullv  formed  extends  from  in  front  of  the  foramen 
of  Monro  to  the  tip  01  the  temporal  lol)C  (I-Ik-  228.  h).     It 
affects  the  entire  thickness  of  the  pallial  wall  and  conse- 
(lucntly  produces  an  elevation  upon  the  inner  surface,  a 
projection  into  the  cavity  of  the  ventricle  which  is  known 
as    the    hippocampus, 
whence      the      fissure 
may   be    termed    the 
hippocampal      fissure. 
The    portion    of     the 
pallium    which    inter- 
venes    between     this 
fissure  and  the  chori- 
oidal    forms    what    is 
known  as  the   dentate 
gyrus. 

Toward  the  end  of 
the  third  or  the  Ix'- 
ginning  of  the  fourth 
month  two  prolonga- 
tions arise  from  the  fissure  just  where  it  turns  to  be 
continued  into  the  temporal  lobe,  and  these,  extending 
posteriorly,  give  rise  to  the  parieto-occipital  and  cnleanne 
fissures.  Like  the  hippocampal,  these  fissures  produce 
elevations  upon  the  inner  surface  of  the  pallium,  that 
formed  by  the  parieto-occipital  early  disappearing,  while 
that  produced  by  the  calcarine  persists  to  form  the  calcar 
(hippocampus  minor)  of  adult  anatomy. 

The  three   fissures  just  described,   together  with   the 


I'lc,  226.     Hkain  (»•  AN  r.MHRVo  in-  tmu 

I'oiKTii  Month. 
r,  Cerehdluin ;    />,  iiniis;    s  Sylvian  f()ss;i. 


!fl 


'^m^'^s^3B!W''-^7''immm:^ 


424 


THE    DEVELOPMENT   OK   THE    HUMAN    BOOV. 


chorioidal  ami  the  fossa  of  Sylvius,  art-  all  formed  by  tlu- 
iK'jrinniuK  of  the  fourth  month  and  all  aiTect  the  entin- 
thickness  of  the  wall  of  the  hemisphere,  and  hence  have 
been  termed  the  prittuny  or  iolal  fissures.  Until  the  be^in- 
niiiK  of  the  fifth  month  they  are  the  (mly  fissures  present, 
but  at  that  time  secondary  fissures,  which,  with  one  excep- 
tion, are  merely  furrows  of  the  surface  of  the  pallium, 
make  their  appearance  and  eoniinue  to  form  until  birth 
and  possiblv  later.  Before  considerinij  these,  however, 
certain  cliani,'es  which  occur  in  the  neighborhood  of  the 
Sylvian  fossa  may  be  describetl. 

The  fossa  is  at  first  a  triangular  depression  situated 
above  the  temporal  lobe  on  the  surface  of  the  hemisphere. 
During'  the  fourth  month  it  deepens  c(msiderably,  so  that 
its  upper  and  lo\\er  margins  become  more  pronotmced 
and  form  projecting  folds,  and.  during  the  fifth  month, 
these  two  folds  approach  one  another  and  eventually 
cover  in  the  floor  of  the  fossa  completely,  the  groove 
which  marks  the  line  of  their  contact  forming  tlie  Sylvian 
fissuye,  while  the  floor  of  the  fossa  becomes  known  as  the 
island  (if  Kc'il  (insula). 

The  first  of  the  secondary  fissures  to  appear  is  the  cal- 
loso-marqinal,  which  is  formed  about  the  middle  of  the 
fifth  month  on  the  mesial  surface  of  the  hemispheres, 
lying  parallel  to  the  anterior  portion  of  the  hippocampus 
fissure   and   dividing  the   mesial   surface   into   the   gyri 
margimilis  and  jornicatus.     A  little  later,  at  the  beginning 
of  the  sixth  month,  several  other  fissures  make  their  ap- 
pearance upon  the  outer  surface  of  the  pallium,  the  chief 
of  these  being  the  fissure  of  Rolando,  the  intra-parietal, 
the  pre-  and  post-central,  and  the  temporal  fissures,  the 
most  ventral  of  these  last  running  parallel  witli  the  lower 
portion  of  the  hippocampal  fissure  and  differing  from  the 
others  in  forming  a  ridge  on  the  wall  of  the  ventricle 


IIIK   CORPUS  c.M.i.osrM. 


425 


tcriiK-(l  the  collateral  nnimncc,  wluucc  the  fissure  is 
known  as  the  collateral,  ihe  position  of  most  of  these 
fissures  may  be  se'«n  from  Fij,'.  227.  iintl  for  a  more  com- 
plete description  01  them  reference  may  be  had  to  text- 
books of  descriptive  anatomy. 

In  later  sta.ijcs  numerous  tertiary  fissures  make  their 
appearance  and  mask  more  or  less  extensively  the  sec- 


Fic.  227.-  Cerebral  Hemim-hkre  ok  .\n  I'mbrvo  of  .\boi  t  the  Seventh 

Month. 
f    SuiJeriur  frontal  fissure;  ip,  intraparietal ;  IK,  island  of  Keil;  ^/,  in- 
ferior   prc-cenlral;    pes,  superior    j.re  central;  ptc,   jiost- central ;   A, 
Rolandic;  S.  Sylvian;  t' ,  first  temporal.     {Citntini^ham.) 

ondaries,  than  which  they  are,  as  a  rule,  much  more  incon- 
stant in  position  and  shallower. 

The  Corpus  Callosum  and  Fornix.— While  these  fissures 
have  been  forming,  important  structures  have  developed 
in  connection  with  the  lamina  terminalis.  Up  to  about 
the  fourth  month  the  lamina  is  thin  and  of  nearly  uniform 
thickness  throughout,  but  at  this  time  it  begins  to  thicken 
at  its  dorsal  edge  to  form  a  mass  which  is  triangular  in 
36 


ili 


i 


-.,  M 


426 


TIIK    nF.VKI.Ol'MKNT    OF   THE    HI  MAN    IJOOV. 


section  and  connects  the  mesial  surfaces  of  the  two  hemi- 
spheres. The  ventral  angle  of  the  thickening  later  sepa- 
rates slightly  from  the  rest  and  fibers  appear  in  it,  con- 
verting it  into  the  anterior  commissure,  and  the  remainder 
of  the  thickening,  continuing  to  increase  in  size  with  the 
increase  of  the  hemispheres,  forms  a  mass  of  considerable 

size,  still  retaining  its 
trial  .,ular  shape   and 
having    its    apex    di- 
rected posteriorly.    In 
the  dorsal  portion  of 
the  triangle  fibers  ex- 
tend across  from  the 
pallium  of  one  hemi- 
sphere to  that  of  the 
other   and    form    the 
corpus  callosutn,  while 
in    its    ventral    edge 
other     fibers     extend 
from  the  hippocampus 
to  the  lamina  termin- 
alis,    and,    descending 
in  that  structure,  pass 
posteriorly  in  the  floor 
of   the  third  ventricle 
toward     the    corpora 
albicantia.      These  fi- 
bers constitute  the  pillars  of  the  fornix,  whose  peculiar 
course  in  the  adult  brain  may  be  understood  by  a  con- 
sideration of   the   rotation  of   the  hemispheres  during 
growth  which  results  in  the  formation  of  the  temporal 
lobe  (seep.  421). 

The  portion  of  the  triangle  included  between  the  callo- 
sum  and  the  fornix  remains  thin  and  forms  the  septum 


Fig.  228. — Medi.xn    Longitudinal  Sec- 
tion  OF   THE   Hr.MN  of  AN  EmBRYO  OF 

Three  Months. 
c,  Calcarine  fissure;  ca,  anterior  commis- 
sure ;  cc,  corpus  callosum ;  cf,  chorioidal 
fissure ;  dg,  dentate  gyrus ;  fm,  foramen 
of  Monro;  h,  hippocampal  fissure;  po, 
parieto-occipital  fissure.  —  (  Mihalko- 
vciz.) 


THE    OLFACTORY    I.OIJES. 


427 


6l 

ac 


lucidum,  and  a  split  occurring  in  the  center  of  this  gives 
rise  to  the  so-called  fifth  ventricle,  which,  from  its  mode 
of  formation,  is  a  completely  closed  cavity  and  is  not 
lined  with  ependymal  tissue  of  the  same  nature  as  that 
found  in  the  other  ventricles. 

Owing  to  the  very  considerable  size  reached  by  the 
thickening  of  the  lamina  terminalis  whose  history  has  just 
been  described,  important  changes  are  wrought  in  the 
adjoining  portions   of 

the  mesial  surface  of  ^„.,-r-4-».w     ''•-' 

the  hemispheres.  Be- 
fore the  development 
of  the  thickening  the 
gyrus  dentatus  and 
the  hippocampus  ex-  ^ 
tend  forward  into  the 
anterior  portion  of 
the  hemispheres  (Fig. 
228),  but  on  account 
of  their  position  they 
become  encroached 
upon  by  the  enlarge- 
ment of  the  lamina 
terminalis,  with  the 
result  that  the  hippo- 
campus becomes  practically  obliterated  in  that  portion 
of  its  course  which  lies  in  the  region  occupied  by  the  cor- 
pus callosum,  its  fissure  in  this  region  becoming  known 
as  the  callosal  fissure,  while  the  corresponding  portions 
of  the  dentate  gyrus  become  reduced  to  narrow  and  in- 
significant bands  of  nerve-tissue  which  rest  upon  the  upper 
surface  of  the  corpus  callosum  and  are  known  as  the  strice 
of  Lancisi. 

The  Olfactory  Lobes. — At  the  time  when  the  cerebral 


Fig.  229. — Median  Longitudinal  Sec- 
tion OF  THE  Brain  of  an  Embryo  of 
THE  Fifth  Month. 

ac.  Anterior  commissure;  cc,  corpus  cal- 
losum; dg,  dentate  gyrus;  /,  fornix; 
J,  infundibulum ;  mc,  middle  commis- 
sure; si,  septum  lucidum;  -vi,  velum 
interpositum, — {Mihalkovicz.) 


fi 


V\  '>\ 


i 

1 

^ 

! 

1 1 

J 1 

19 

\ 

I 

I 


428 


THE    DEVELOPMENT    OF     THE    HUMAN    BODY. 


I 


hemispheres  begin  to  enlarge— that  is  to  say,  at  about  the 
fourth  week— a  slight  furrow,  which  appears  on  the  ven- 
tral surface  of  each  anteriorly,  marks  off  an  area  which, 
continuing  to  enlarge  with  the  hemispheres,  gradually 
becomes  constru  ed  off  from  them  to  form  a  distinct 
lobe-like  structure,  the  olfactory  lobe  (Fig.  2 1 7,  VI  3)-     I" 
most  of  the   lower  mammalia  these  lobes  reach  a  very 
considerable  size,  and  consequently  have  been  reg:irdcd 
as  constituting  an  additional  division  of  the  brain,  known 
as  the  rhinencephalon,  but  in  man  they  remain  smaller, 
and  although  they  are  at  first  hollow,  containing  pro- 
longations from  the  lateral  ventricles,  the  cavities  later 
on  disappear  and  the  lobes  become  solid.     Kacli  lobe 
becomes  differentiated  into  two  portions,  its  terminal  por- 
tion becoming  converted  into  the  club-shaped  structure, 
the  olfactory  bulb  and  stalk,  while  its  proximal  portion 
gives  rise  to  the  olfactory  tracts,  the  trigone,  and  the 
anterior  perforated  space. 

Histogenesis  of  the  Cerebral  Cortex.— \  satisfactory  study 
of  the  histogenesis  of  the  cortex  has  not  yet  been  made. 
In  embryos  of  three  months  a  marginal  velum  is  present 
and  probably  gives  rise  to  the  stratum  zonale  of  the  adult 
brain ;  beneath  this  is  a  cellular  layer,  perhaps  represent- 
ing the  mantle  layer;  beneath  this,  again,  a  layer  of  nerve- 
fibers  is  beginning  to  appear,  representing  the  white  sub- 
stance of  the  pallium;  and,  finally,  lining  the  ventricle  is 
an  ependymal  layer.  In  embryos  of  the  fifth  month 
toward  the  innermost  part  of  the  second  layer  cells  are 
beginning  to  differentiate  into  the  large  pyramid  cells, 
but  almost  nothing  is  known  as  to  the  origin  of  the  other 
layers  recognizable  in  the  adult  cortex,  nor  is  it  known 
whether  any  migration,  similar  to  what  occurs  in  the  cere- 
bellar cortex,  takes  place.  The  fibers  of  the  white  sub- 
stance do  not  begin  to  acquire  their  myelin  sheaths  until 


THE    SPINAL    NERVES. 


429 


toward  the  end  of  the  ninth  month,  and  the  process  is  not 
completed  until  some  time  after  birth  (Flechsig),  while 
the  fibers  of  the  cortex  continue  to  undeij^o  myelination 
until  comparatively  late  in  life  (Kaes). 

The  Development  of  the  Spinal  Nerves. — It  has  already 
been  seen  that  there  is  a  fundamental  diff^'-ence  in  the 
mode  of  development  of  the  two  roots  of  which  the  typical 
spinal  nerves  are  composed,  the  ventral  root  being  formed 
by  axis-cylinders  which  arise  from  neuroblasts  situated 
within  the  substance  of  tlie  spinal  cord,  while  the  dorsal 
roots  arise  from  the  cells  of  the  U'  ural  crests,  their  axis- 
cylinders  growing  into  the  substance  of  the  cord  wh'le 
their  dendrites  become  prolonged  peripherally  to  form 
the  sensory  fibers  of  the  nerves,  'throughout  the  thoracic, 
lumbar  and  sacral  regions  of  the  cord  the  fibers  which 
■  i  ■'  out  from  the  anterior  horn  cells  converge  to  form 
I  igle  nerve-root  in  each  segment,  but  in  the  cervical 
.  on  the  fibers  which  arise  from  the  more  laterally  situ- 
ated neuroblasts  make  their  exit  from  the  cord  inde- 
pendently of  the  more  ventral  neuroblasts  and  form  the 
roots  of  the  spinal  accessory  nerve  (see  p.  438).  In  the 
cervical  region  there  are  accordingly  three  sets  of  nerve- 
roots,  the  dorsal,  lateral,  and  ventral  sets,  the  last  being 
not  quite  equivalent  to  the  similarly  named  roots  of  the 
more  posterior  nerves. 

In  a  typical  spinal  nerve,  such  as  one  of  the  thoracic 
series,  the  dorsal  roots  as  they  grow  peripherally  pass 
downward  as  well  as  outward,  so  that  they  quickly  come 
into  contact  with  the  ventral  roots  with  whose  fibers  they 
mingle,  and  the  mixed  nerve  so  formed  soon  after  divides 
into  two  trunks,  a  dorsal  one,  which  is  distributed  to  the 
dorsal  musculature  and  integument,  and  a  larger  ventral 
The  ventral  division  as  it  continues  its  outward 


14 


one. 


growth  soon  reaches  the  dorsal  angle  of  the  pleuro-peri- 


i     i 


430 


THE    DEVELOPMENT    OF    THE    HUMAN    BODY, 


toneal  cavity,  where  it  divides,  one  branch  passing  into 
the  tissue  of  the  body- wall  while  the  other  passes  into  the 
splanchnic  mesoderm.  The  former  branch,  continuing 
its  onward  course  in  the  body-wall,  again  divides,  one 
branch  becoming  the  lateral  cutaneous  nerve,  while  ihe 
other  continues  inward  to  terminate  in  the  median  ven- 
tral portir-'.'  of  the  body  as  the  anterior  cutaneous  nerve. 
The  splanchnic  branch  forms  a  ramus  communicans  to 
the  sympathetic  system  and  will  be  considered  more 
fully  later  on. 

The  conditions  just  described  are  those  which  obtain 
throughout  the  greater  part  of  the  thoracic  region.  Else- 
where the  fibers  of  the  vtntral  divisions  of  the  nerves  as 
they  grow  outward  tend  to  separate  from  one  another  and 
to  become  associated  with  the  fibers  of  adjacent  nerves, 
giving  rise  to  plexuses.  In  the  regions  where  the  limbs 
occur  the  formation  of  the  plexuses  is :  Iso  associated  with 
a  shifting  of  the  parts  to  which  the  nerves  are  supplied,  a 
factor  in  plexus  formation  which  is,  however,  much  more 
evident  from  comparative  anatomical  than  from  embry- 
logical  studies. 

The  Development  of  the  Cranial  Nerves.— During  the 
last  thirty  years  the  cranial  nerves  have  received  a  great 
deal  of  attention  in  connection  with  the  idea  that  an 
accurate  knowledge  of  their  development  would  afford 
a  clue  to  a  most  vexed  problem  of  vertebrate  morphology, 
the  metamerism  of  the  head.  That  the  metamerism 
which  was  so  pronounced  should  extend  into  the  head  was 
a  natural  supposition,  strengthened  by  the  discovery  of 
head-cavities  in  the  lower  vertebrates  and  by  the  indica- 
tions of  metamerism  seen  in  the  branchial  arches,  and  the 
problem  which  presented  itself  was  the  correlation  of  the 
various  structures  belonging  to  each  mctamcrc  and  the 
determination  of  the  modifications  which  they  had  under- 
gone during  the  evolution  of  the  head. 


THE    CRANIAL    NERVKS. 


431 


In  the  trunk  region  a  nerve  forms  a  conspicuous  ele- 
ment of  each  metamere  and  is  composed,  according  to 
what  is  known  as  Bell's  law,  of  a  ventral  or  efferent  and 
a  dorsal  r  afferent  root.  Until  comparatively  recently 
the  study  of  the  cranial  nerves  has  been  dominated  by  the 
idea  that  it  was  possible  to  extend  the  application  of  Bell's 
law  to  them  and  to  recognize  in  the  cranial  region  a  num- 
ber of  nerve  pairs  serially  homologous  with  the  spinal 
nerves,  some  of  them,  however,  having  lost  their  afferent 
roots,  while  in  others  a  dislocation,  as  it  were,  of  the  two 
roots  had  occurred. 

The  results  obtained  from  investigation  along  this  line 
have  not,  however,  proved  entirely  satisfactory,  and  facts 
have  been  elucidated  which  seem  to  show  that  it  is  not 
possible  to  extend  Bell's  law,  in  its  original  form  at  least, 
to  the  cranial  nerves.  It  has  been  found  that  it  is  not 
sufficient  to  recognize  simply  afferent  and  efferent  roots, 
but  these  must  be  analyzed  into  further  components,  and 
when  this  is  done  it  is  found  that  in  the  series  of  cranial 
nerves  certain  components  occur  which  are  not  repre- 
sented in  the  nerves  of  the  spinal  series. 

Before  proceeding  to  a  description  of  these  components 
it  will  be  well  to  call  attention  to  a  matter  already  alluded 
to  in  a  previous  chapter  (p.  127)  in  connection  with  the 
segmentation  of  the  mesoderm  of  the  head.  It  has  been 
oointed  out  that  while  there  exist  "head-cavities"  which 
are  serially  homologous  with  the  mesodermal  somites  of 
the  trunk,  there  has  been  impose -•.  upon  this  primary 
cranial  metamerism  a  secondary  metamerism  represented 
by  the  branchiomeres  associated  with  the  branchial 
arches,  and,  it  may  be  added.'this  secondary  metamerism 
has  become  the  more  prominent  of  the  two,  the  primary 
one,  as  it  developed,  gradually  slipping  into  the  back- 
ground until,  in  the  higher  vertebrates,  it  has  become  to 


432 


THE  DEVEI.OPMENT  OF  THE  HUMAN  BODY. 


i 


a  very  considerable  extent  rudimentary.  In  accordance 
with  this  double  metamerism  it  is  necessary  to  recognize 
two  sets  of  cranial  muscles,  one  derived  from  the  cranial 
myotomes  and  represented  by  the  muscles  of  the  eyeball, 
and  one  derived  from  the  branchiomeric  mesoderm,  and 
it  is  necessary  also  to  recognize  for  these  two  sets  of  mus- 
cles two  sets  of  motor  nerves,  so  that,  with  the  dorsal  or 


Fig.   230.— Transvkrse  Section  through  the  Medulla  Oblongata 
OK  AN  Embryo  of  10  mm.,  showing  the  Nuclei  of  Origin  of  the 

\'AGUS  (A)   AND  HvPtMJLOSSAL  (A7/)   NERVES.— (//W.) 

sensory  nerve-roots,  there  are  altogether  three  sets  of 
nerve-roots  in  the  cranial  region  instead  of  only  two,  as  in 
the  spinal  region. 

These  three  sets  of  roots  are  readily  recognizable  both 
in  tlie  embryonic  and  in  the  adult  brain,  especially  if  at- 
tention be  directed  to  the  cell  groups  or  nuclei  with  which 
they  are  associated  (Fig.  230).     Thus  there  can  be  recog- 


THE  CKANIAL  NKRVKS. 


433 


nized :  ( i )  a  series  of  nuclei  from  which  ncrve-fibers  arise, 
situated  in  the  floor  of  the  fourth  ventricle  and  iter  close 
to  the  median  line  and  termed  the  ventral  motor  nuclei ; 
(2)  a  second  series  of  nuclei  of  origin,  situated  more  lat- 
erally and  in  the  substance  of  the  formatio  reticularis,  and 
known  as  the  lateral  motor  nuclei;  and  (3)  a  series  of 
nuclei  in  which  afferent  nerve-fibers  terminate,  situated 
still  more  laterally  in  th,  floor  of  the  ventricle  and  forming 
the  dorsal  or  sensory  nuclei.     None  of  the  twelve  cranial 
nerves  usually  recognized  in  the  textbooks  contain  fibers 
associated  with  all  three  of  these  nuclei;  the  fibers  from 
the  lateral  motor  nuclei  almost  invariably  unite  with  sen- 
sory fibers  to  form  a  mixed  nerve,  but  those  from  all  the 
motor  nuclei  form  independent  roots,  while  the  olfactory 
and  auditory  nerves  alone,  of  all  the  sensory  roots  (omit- 
ting for  the  present  the  optic  nerve),  do  not  contain  fibers 
from  either  of  the  series  of  niotor  nuclei.     The  relations 
of  the  various  cranial  nerves  to  the  nuclei  may  be  seen 
from  the  following  table,  in  which  the   f  sign  indicates 
the  presence  and  the  —  sign  the  ubsence  of  fibers  from  the 
nuclear  series  under  which  it  stands : 


Number. 

Namk. 

I. 

Olfactory. 

III. 

Oculomotor. 

IV. 

Trochlear. 

V. 

Trigeminus. 

VI. 

Ahduccns. 

VII. 

Facial. 

VIII. 

Auditory. 

IX. 

Glossopharyngeal. 

X. 

Vagus.                      \ 

XI. 

Spinal  Accessory.  J 

Ventral 
Motor. 

Lateral 
Motor. 





-*- 

z 

+ 

-1 

4- 

— 

+ 

• — 

+ 

Sensory. 


+ 


+ 

+ 

I 

-r 

+ 
-t- 


Two  nerves — namely,  the  second  and  twelfth — have 
been  omitted  from  the  above  table.  Of  these,  the  second 
or  opti  J  nerve  undoubtedly  belongs  to  an  entirely  differ- 


Ml; 


91     If 
■!     ! 


','1')  I 


4 


F  ! 


434 


THE    DEVELOPMENT    OF   THE    HL'MAN    IJOPY. 


ent  category  from  the  other  peripheral  nerves,  and  will  be 
considered  in  the  following  chapter  in  connection  with  the 
sense-organ  with  which  it  is  associated  fsec  especially 
p.  489).  The  twelfth  or  hypoglossal  nerve,  on  the  other 
hand,  really  belongs  to  the  spinal  series  and  has  only  sec- 
ondarily been  taken  up  into  the  cranial  region  in  the 
higher  vertebrates.  It  has  already  been  seen  (p.  192)  that 
the  bodies  of  four  vertebrae  are  included  in  the  basioccipi- 
tal  bone,  and  that  three  of  the  nerves  corresponding  to 
these  vertebrae  are  represented  in  the  adult  by  the  hypo- 
glossal and  the  fourtii  by  the  first  cervical  or  suboccipital 
nerve.  The  dorsal  roots  of  the  hypoglossal  nerves  seem  to 
have  almost  disappeared,  although  a  ganglion  has  been 
observed  in  embryos  of  7  and  10  mm.  in  the  posterior 
part  of  the  hypoglossal  region  (His),  and  probably  repre- 
sents the  dorsal  root  of  the  most  posterior  portion  of  the 
hypoglossal  nerve.  Tliis  ganglion  disappears,  as  a  rule,  in 
later  stages,  and  it  is  interesting  to  note  that  the  ganglion 
of  the  suboccipital  nerve  is  also  occasionally  wanting  in 
the  adult  condition.  The  hypoglossal  roots  are  to  be  re- 
garded, then,  as  equivalent  to  the  ventral  roots  of  the 
cervical  spinal  nerves,  and  the  nuclei  from  which  they 
arise  lie  in  series  with  the  cranial  ventral  motor  roots,  a 
fact  which  indicates  the  equivalency  of  these  latter  with 
the  fibers  which  arise  from  the  neuroblasts  of  the  anterior 
horns  of  the  spinal  cord. 

The  equivalents  of  the  lateral  motor  roots  may  more 
conv^eniently  be  considered  later  on,  but  it  may  be  pointed 
out  here  that  these  are  the  fibers  which  are  distributed  to 
the  muscles  of  the  branchiomeres.  In  the  case  of  the 
sensory  nerves  a  further  analysis  is  necessary  before  their 
equivalents  in  the  spinal  series  can  be  determined.  For 
this  the  studies  which  have  been  made  in  recent  years  of 
the  components  entering  into  the  cranial  nerves  of  the 


i 

M. 


TIIK    CRAN'IAr,    NKRVKS. 


435 


amphibia  (Strong)  and   fishes  (Herrick)  must  supply  a 
basis,  since  as  yet  a  direct  analysis  of  the  mammalian 
nerves  has  not  been  made.    In  the  forms  named  it  has  been 
found  that   three  different  components  enter  into  the 
formation  of  the  dorsal  roots  of  the  cranial  nerves:  (i) 
fibers  belonging  to  a  general  cutaneous  or  somatic  sensory 
system,  distributed  to  the  skin  without  being  connected 
with  any  special  sense-organs;    (2)  fibers  belonging  to 
what  is  termed  the  communis  or  viscerosensory  system, 
distributed  to  the  walls  of  the  mouth  and  pharyngeal 
region  and  to  special  organs  found  in  the  skin  of  the  same 
character  as  those  occurring  in  the  mouth;  and  (3)  fibers 
belonging  to  a  special  set  of  cutaneous  sense-organs  largely 
developed  in  the  fishes  and  known  as  the  organs  of  the 
lateral  line. 

The  fibers  of  the  somatic  sensory  system  converge  to  a 
group  of  cells,  situated  in  the  lateral  part  of  the  floor  of 
the  fourth  ventricle,  and  forming  what  is  termed  the  tri- 
geminal lobe,  and  also  extend  posteriorly  in  the  substance 
of  the  medulla  (Fig.  231),  forming  what  has  been  termed 
the  ascending  root  of  the  trigeminus  and  terminating  in  a 
column  of  cells  which  represents  the  forward  continuation 
of  the  posterior  horn  of  the  cord.     In  the  fishes  and  am- 
phibia fibers  belonging  to  this  system  are  to  be  found  in 
the  fifth,  seventh,  and  tenth  nerves,  but  in  the  mamma- 
lia their  distribution  has  apparently  become  more  limited, 
being  confined  almost  exclusively  to  the  trigeminus,  of 
whose  sensory  divisions  they  form  a  very  considerable  part. 
vSince  the  cells  around  which  the  fibers  of  the  ascending 
root  of  the  trigeminus  terminate  are  the  forward  continua- 
tions of  the  posterior  horns  of  the  cord,  it  seems  probable 
that  the  fibers  of  this  system  are  the  cranial  representa- 
tives of  the  posterior  roots  of  the  spinal  nerves,  which, 
it  may  be  noted,  are  also  somatic  in  their  distribution. 


M 


436 


THE    DEVELOPMENT   OF   THE    HUMAN    UODV. 


The  fibers  of  the  viscerosensory  system  are  found  in  the 
lower  forms  principally  in  the  ninth  and  tenth  nerves  (see 
Fig.  231),  although  groups  of  them  are  also  incorpor- 
ated in  the  seventh  and  fifth.  They  converge  to  a  mass 
of  cells,  known  as  the  lobus  vagi,  and  like  the  first  set 
are  also  continued  down  the  medulla  to  loim  a  tract 


rix 


Fig.  231. — Diagram  showing  thb  Sknsory  Components  of  the  Crani.al 
Nerves  of  a  Fish  {Mcnidia). 

The  somatic  sensory  system  is  unshaded,  the  viscero-sensory  is  cross- 
hatched,  and  the  lateral  line  system  is  black,  asc.v,  Ascending  root 
of  trigeminus;  hrx,  branchial  branches  of  vagus;  ol,  olfactory  bulb; 
op,  optic  nerve;  rc.x,  cutaneous  branch  of  the  vagus;  rix,  intestinal 
branch  of  vagus;  W,  lateral  line  nerve;  rl.acc,  accessory  lateral  line 
nerve ;  ros,  superficial  ophthalmic ;  rp,  ramus  palatinus  of  the  facial ; 
thy,  hyoniandibular  branch  of  the  facial;  t.inf,  infraorbital  nerve. — 
{Herrick.) 


f    I 


known  as  the  fasciculus  solitarius  or  fasciculus  communis. 
In  the  mammalia  the  system  is  represented  by  the  sensory 
fibers  of  the  glossopharyngeo-vagus  set  of  nerves,  of  which 
it  represents  practically  the  entire  mass;  by  the  sensory 
fibers  of  the  facial  arising  from  the  geniculate  ganglion  and 
included  in  the  chorda  tympani  and  probably  also  the 


THE  CRANIAL  NERVES. 


437 


great  superficial  petrosal;  ami  also,  probably,  by  the  lin- 
gual branch  of  the  trigeminus.  Furthermore,  since  the 
mucous  membrane  of  the  palate  is  supplied  by  branches 
from  the  trigeminus  which  pass  by  way  of  the  spheno- 
palatine (Meckel's)  ganglion,  and  the  same  region  is  sup- 
plied in  lower  forms  by  a  palatine  branch  from  the  facial, 
it  seems  probable  that  the  palatine  nerves  of  the  mam- 
malia are  also  to  be  assigned  to  this  system.*  If  this  be 
the  case,  a  very  evident  clue  is  afforded  to  the  homologies 
of  the  system  in  the  spinal  nerves,  for  since  the  spheno- 
palatine ganglion  is  to  be  regarded  as  part  of  the  sympa- 
thetic system,  the  sensory  fibers  which  pass  from  the  vis- 
cera to  the  spinal  cord  by  way  of  the  sympathetic  system 
(p.  443)  present  relations  practically  identical  with  those 
of  the  palatine  nerves. 

Finally,  with  regard  to  the  system  of  the  lateral  line, 
there  seems  but  little  doubt  that  it  has  no  representation 
whatsoever  in  the  spinal  nerves.  It  is  associated  with  a 
peculiar  system  of  cutaneous  sense-organs  found  only  in 
aquatic  or  marine  animals,  and  also  with  the  auditory 
and  possibly  th*-  olfactory  organs,  the  former  of  which  are 
certainly  and  the  latter  possibly  primarily  parts  of  the 
lateral  line  system  of  organs.  The  organs  are  principally 
confined  to  the  head,  although  they  also  extend  upon  the 
trunk,  where  they  are  followed  by  a  branch  from  the 
vagus  nerve,  the  entire  system  being  accordingly  supplied 
by  cranial  nerves.  In  the  fishes,  in  which  the  develop- 
ment of  the  organs  is  at  a  maximum,  fibers  belonging  to 
the  system  are  found  in  all  the  brancliiomeric  nerves  and 


*  The  fact  that  the  palatine  branches  are  associated  with  the  tri- 
geminus in  the  Mammalia  and  with  the  facial  in  the  Amphibia  is  readily 
explained  by  the  fact  that  in  the  latter  the  Gasserian  and  geniculate 
ganglia  are  not  always  separated,  so  that  it  is  possible  for  fi^  :s  origi- 
nating from  the  compound  ganglion  to  pass  into  either  nerve. 


:l 


^Wr.  I; 


438 


TIIK    DKVKI.OI'MKN  I 


1  iii 


Ml   »' 


i;oi)V. 


all  converge  to  a  portion  t*  Hit  nudull;!  known  as  tlic 
tubcrciilum  acusticuw  In  th  -  Ma-;r  ''lia,  with  tlic  disap- 
pearance of  tlic  lateral  line  '>rj,\ans  tin. it  has  been  a  disap- 
pearance of  the  associated  nerves,  and  the  only  represen- 
tatives of  the  system  whic  h  persist  are  the  auditory  and 
olfactory  nerves. 

The  table  ,i,dven  on  page  4,^3  may  now  be  expanded  as 
follows,  though  it  nuist  be  itcognized  that  such  an  analy- 
sis of  the  mannnalian  nerves  is  merely  a  (k-duction  from 
what  has  been  observed  in  lower  forms,  and  may  recjuire 
some  modifications  when  the  ci  inponents  have  be»»n  sub 
jected  to  actual  observation : 

^ ^---iu.  


Nkkve. 

v. 

Nl  RAl, 

I.ATKRAI. 

Somatic 

Vist  I'k  Al. 

l,A  1  1  KAL 

M 

UIOK. 

.Mo  I  OR. 

Sk.nmikv. 

Sknsiiky. 

Link. 

I. 
III. 

-f 



! 

— 

-f 

IV. 

+ 



— 

— 

V. 



+• 

-f- 

— 

VI. 

-f 

— . 

-_ 

— . 

— 

VII. 

-f 

1       -— 

-f- 

— 

VIII. 

. 

:           — 

-  - 

-f 

IX.) 

1 

X. 

— 

-i- 

+ 

XI. j 

XII. 

-i- 



— . 



-^ 

Spinal. 

i 

-}- 

{') 

r 

I              + 

■ — 

In  # 


I 


ii       i 


An  additional  word  is  necessary  conccrnin<^  the  spinal 
accessory  nerve,  for 'it  present  certain  interesiing  rela 
tions  which  possibly  furnish  a  chic  to  the  spinal  equi-  i 
lents  of  the  lateral  motfir  root>.  In  the  first  ;,  lace,  w 
neuroblasts  which  give  rise  to  those  hhers  of  t  .e  ner\e 
which  coHi  from  the  spinal  cord  are  situated  in  ii  -c  <-ursaI 
part  of  the  ventral  zones  and  in  the  adult  in  the  lai  ra! 
horn  of  the  cord.  As  the  nuclei  of  origin  are  traced  ai  e- 
riorly  they  will  be  found  to  change  their  position  sor  e- 


I  III      (  KANI  \l.    N|;K\  Is. 


439 


wli  it  as  tin  iiu  <lulla  is  rcaclii  i  uiul  <      ntually  conir  to  lie 
iiithf  reticular  I  trtuatioii.  tin  most       terior  of  them  Ik'IU.^ 
practicallv    coniimious   with     lie         tor  r  .cleus  of  th 
va.i,'us.     lud'  I  cl.    it   siii  IS  prohubk'   that      ^tain  nerve 
roots  Ih  loiiijiiiK'  1  >  the  vaL.us  s«      whiih  occui  in  the  lower 
vertcliratcs  inini.  diately  hehiiKi  the  motor  roots  of  the 
va^ns  and  ai     i^rtii  d  tin-  s{>i no-occipital  nerves   dnir 
hriiit^cr),  are  i!ic<)rporate(l  in  the  spinal  accessory  of  iiij^her 
forms  and  constitn**  the  portion  of  that  nerve  vhich  si'p 
plies  tin      erno  tiKisioid  and  trai)ezins  nuisrU's. 

It  IS  believed  diat  tin  whi'v-  rami  ci-  nmitnicantv.?. 
wliich  pass  from  tin  spinal  cord  to  the  thoracic  a  cl  upper 
lumbar  -.ynipathelic  K^i'i^iia  aris.  from  cells  situ  .t?  1  in 
thf  dor-o-lateral  portion^  '  the  ntral  Iif)ms.  an  aice 
ihc'-v  rami  are  lackintjin  tin  riL,ioi!  in  wlr  h  tin  aalu<'- 
(■cs-.*irvo(vnrs,  it  would  seem  that  this  ncrvt  ma  r  reseat 
the  w'.itc  rami  (j1"  the  cervical  segments  Th  t(  atationis 
„ri  ft  to  carr  this  ;ne  of  ..omohj^^y  to  its  c  ts  usit  n,  and 
to  rej?ard  llic  .rainul  lateral  motor  rof  ^~  as  t^mivalent  to 
the  wdiite  rami  •"  use  cord,  and  the  t'  nij  tati*  •  intensi- 
fied when  it  is  :ecah'ed  that  there  art,  hr;  ii  t  mbrvol  ti,dcal 
ami  topograj diica!  reasons  for  regardii  -g  '  ie  I  mchionieric 
jnuscles,  to  ^  '  'oli  the  cranial  lateral        >t  ves  are 

ipplird,  as    jquivalent  t      the  viscerr  >  ies  of  the 

iiunk.     But    n  \\'^-\\  of  the  fai     that  a  s  n  tic  neu- 

ron* i^  ahvays  interpose  :  be  vvecii .  vhite  aiii  s  liber  and 
the  viscf-ral  ii  iiscidature,  while  *liv  lateral  motor  fibers 
connect  directlv  with  the  branchionuric  musculature,  it 
seems  advisable  to  aw  lit  further  studies  before  yielding 
-to  the  temptatit    1. 

Af.  regards  the  actv  1  development  of  th«  cranial  nerves, 
the  follow  the  gene  d  law  which  obtains  for  the  spinal 
nerves,  the  motor  fibers  being  outgrowths  from  neuro- 
blasts situated  in  the  walls  of  the  neural  tube,  while  the 


iff 


Wi 


:i 


1  i''^ 


>  i.i  ■ 


L 


440  THE    nEVELOPMENT   OF    THE    HUMAN    BODY. 

sensory  nerves  are  outgrowths  from  the  cells  of  ganglia 
situated  without  the  tube.     In  the  lower  vertebrates  a 
series  of  ganglia,  known  as  the  supr abranchial  ganglia,  are 
developed  from  the  ectoderm  along  a  line  corresponding 
with  the  level  of  the  auditory  invagination,  while  on  a  hne 
corresponding  with  the  upper  extremities  of  the  branchial 
clefts  another  series  occurs  which  has  been  termed  that  of 
the  epibranchial  ganglia,  and  with  both  of  these  sets  the 
cranial  nerves  are  in  connection.     In  the  mammalia  these 
structures  have  not  yet  been  sufficiently  studied,  but  from 
the  general  relationship  of  the  suprabranchial  ganglia  it 
seems  probable  that  they  are  associated  with  the  lateral 
line  nerves  and  are  conseciuently  represented  in  the  mam- 
malia only  by  the  ganglia  of  the  auditory  nerve. 

From  what  has  been  said  above  it  is  clear  that  the  usual  ar- 
rangement of  the  cranial  nerves  in  twelve  pairs  does  not  repre- 
sent their  true  relationships  with  one  another.     The  various 
pairs  are  seriallv  homologous  neither  with  one  another  nor  with 
Uie  typical  spinal  nerves,  nor  can  they  be  regarded  as  repre- 
senting twelve  cranial  segments.     Indeed,  it  would  seem  that 
comparativelv  little  information  with  regard  to  the  number  of 
mXmic  segments  which  have  fused  together  to  form  the  head 
S  to  be  deriv?.d  from  the  cranial  nerves,  for  while  there  are  only 
four  of  these  nerves  which  are  associated  with  structures  equiv- 
alent to  the  mesodermic  somites  of  the  trunk,  a  much  greater 
number  of  head  cavities  or  mesodermic  somites  has  been  ob- 
served in  the  cranial  region  of  the  -f^^-^-^l^^^^Z^^Zn 
brates,  Dohrn,  for  instance,  having  found  nineteen  and  Killian 
eSiteen  in  the  cranial  region  of  Torpedo.     Furthermore,  it  is 
mf  I^Sible  to  sav  at  present  whether  the  branch lomeres  and 
their  associated  nerves  correspond  with  one  or  several  of  the 
cranial  mesodermic  somites,  or  whether,   indeed,  any  corre- 
SDondence  whatever  exists.  .  •  .•        i,  „„ 

^n  earlv  stages  of  development  a  series  of  constrictions  have 
been  observed  in  the  cranial  portion  of  the  neural  tube  and  have 
been  regarded  as  indicating  a  primitive  segmentation  of  that 
structure.  The  neuromeres,  as  the  intervals  between  successive 
constrictions  have  been  termed,  seem  to  ^o^^^P^^^J^^.^^  ^^^;. 
cranial  nerves  as  usually  recognized  and  hence  cannot  be  re 


THE   SYMPATHETIC    SYSTEM. 


441 


garded  as  primitive  segmental  structures.  They  are  more 
probably  secondary  and  due  to  the  arrangement  of  the  neuro- 
blasts corresponding  to  the  various  nerves. 

The  Development  of  the  Sympathetic  Nervous  System.— 

From  the  embryological  standpoint  the  distinction  which 
has  been  generally  recognized  between  the  sympathetic 
and  central  nervous  systems  does  not  exist,  the  former 
having  been  founJ  to  be  an  outgrowth  from  the  periph- 
eral ganglia  of  the  latter.  This  mode  of  origin  has  been 
observed  with  especial  clearness  in  the  embryos  of  some 
of  the  lower  vertebrates,  in  which  masses  of  cells  have  been 
seen  to  separate  from  the  posterior  root  ganglia  to  form 
the  ganglia  of  the  ganglionated  cord  (Fig.  232).  In  the 
mammalia,  including  man,  the  relations  of  the  two  sets  of 
ganglia  to  one  another  is  by  no  means  so  apparent,  since 
the  sympathetic  cells,  instead  of  being  separated  from  the 
posterior  root  ganglion  en  masse,  migrate  from  it  singly  or 
in  groups,  and  are  therefore  less  readily  distinguishable 
from  the  surrounding  mesodermal  tissues. 

To  understand  the  development  of  the  sympathetic 
system  it  must  be  remembered  that  it  consists  typically 
of  three  sets  of  ganglia.  One  of  these  is  constituted  by 
the  ganglia  of  the  ganglionated  cord  (Fig.  233,  GC),  the 
second  is  represented  by  the  ganglia  of  the  praevertebral 
plexuses  (PVG),  such  as  the  cardiac,  solar,  hypogastric, 
and  pelvic,  while  the  third  or  peripheral  set  (PG)  is  formed 
by  the  cells  which  occur  throughout  the  tissues  of  proba- 
bly most  of  the  visceral  organs,  either  in  small  groups  or 
scattered  through  plexuses  such  as  the  Auerbach  and 
Meissner  plexuses  of  the  intestine.  Each  cell  in  these 
various  ganglia  stands  in  direct  contact  with  the  axis- 
cylinder  of  a  cell  situated  in  the  central  nervous  system, 
probably  in  the  lateral  horn  of  the  spinal  cord  or  the  cor- 
responding region  of  the  brain,  so  that  each  cell  forms  the 
37 


i 


Fig.  2.^2.— Transversb  Section  through  an  Embryo  Shark  (Scyllium) 

OP  15  MM.,  SHOWING  THE  ORIGIN  OK  A  SYMPAXHETIC  GANGLION. 

Ch.  Notochord ;  E,  ectoderm;  G,  posterior  root  ganRlion;  Gj,  sympathetic 

ganglion;  M,  spinal  cord. — (Onodi.) 

442 


THE    SYMPATHETIC    SYSTEM. 


443 


terminal  link  of  a  chain  whose  first  link  is  a  neurone 
belonging  to  the  central  system  fHuber).  Throughout 
the  thoracic  and  upper  lumbar  regions  of  the  body  tlie 
central  system  neurones  form  distinct  cords  known  as  the 
■white  rami  communicantes  (Fig.  233,  WR),  which  pass 
from  the  spinal  nerves  to  the  adjacent  ganglia  of  the  gan- 
glionated  cord,  some  of  them  terminating  around  the  cells 
of  these  ganglia,  others  passing  on  to  the  cells  of  the  prae- 
vertebral  ganglia,  and  others  to  those  of  the  peripheral 


■I  I 


Fig.  233.— Diagkam  showing  the  Arr.xngement  of  the   Neurones 

OF  THE  SyMP.'VTHETIC  SYSTEM. 

The  fibers  from  the  posterior  root  gansHa  are  represented  by  the  broken 
l)lack  Hnes;  those  from  the  anterior  horn  cells  by  the  sohd  black; 
the  white  rami  by  red;  and  the  sympathetic  neurones  by  blue. 
PR,  Dorsal  ramus  of  spinal  nerve;  GC,  ganglionated  cord;  GR,  gray 
rannis  comnmnicans;  I'G,  peripheral  ganglion;  PVG,  prevertebral 
Kan^'lion;  VR,  ventral  ramus  of  spinal  nerve;  ll'A'.Vhilc  ramus  com- 
nmnicans.— {Adapted  from  Huber.) 

plexuses.  In  the  cervical,  lower  lumbar  and  sacral  regions 
white  rami  are  wanting,  the  central  neurones  in  the  first- 
named  region  probably  making  their  way  to  the  sympa- 
thetic cells  largely  by  way  of  the  spinal  accessory  nerves, 
while  in  the  lower  regions  they  may  pass  down  the  gan- 
glionated  cord  from  higher  regions  or  may  join  the  pra- 
vertebral  and  peripheral  ganglia  directly  without  passing 
through  the  proximal  ganglia.     In  addition  to  these  white 


..f,)i 


■m 


"  ^& 


444 


THE    DEVELOPMENT   OF   THE    HUMAN    BODY. 


1 


rami,  what  are  known  as  gray  rami  also  extend  between 
the  proximal  ganglia  and  the  spinal  nerves;  these  are 
composed  of  fibers,  arising  from  sympathetic  cells,  which 
join  the  spinal  nerves  in  order  to  pass  with  them  to  their 
ultimate  distribution. 

The  brief  description  here  given  applies  especially  to  the 
sympathetic  system  of  the  neck  and  trunk.  Represen- 
tatives of  the  system  are  also  found  in  the  head,  in  the 
form  of  a  series  of  ganglia  connected  with  the  trigeminus 
and  facial  nerves  and  known  as  the  ciliary  (lenticular), 
sphenopalatine,  otic,  and  submaxillary  ganglia;  and,  as 
will  be  seen  later,  there  are  probably  some  sympathetic 
cells  which  owe  their  origin  to  the  root  ganglia  of  the 
pneumogastric  and  glossopharyngeal  nerves.  There  is 
nothing,  however,  in  the  head  region  corresponding  to  the 
longitudinal  bundles  of  fibers  which  unite  the  various 
proximal  ganglia  of  the  trunk  to  form  the  ganglionated 

cord. 

The  first  inHications  of  the  sympathetic  system  are  to  be 
seen  in  a  huuian  embryo  of  about  7  mm.  As  the  spinal 
nerves  reach  the  level  of  the  dorsal  edge  of  the  body- 
cavity,  they  branch,  one  of  the  branches  continuing  ven- 
trally  in  the  f,ody-wall,  wliile  the  other  (Fig.  234, irr)  passes 
mesially  toward  the  aorta,  some  of  its  fibers  reaching  that 
structure,  while  others  bend  so  as  to  assume  a  longitudinal 
direction.  These  mesial  branches  represent  the  white 
rami  communicantes,  but  as  yet  no  ganglion  cells  can  be 
seen  in  their  course.  Tlio  cells  of  the  posterior  root 
ganglia  have  already,  for  the  most  part,  assumed  their 
bipolar  form,  but  among  them  there  may  still  be  found  a 
number  of  cells  in  the  iMiiroblast  condition,  and  these 
(Fig.  234,  s),  wandtring  out  from  the  gaii/lia,  give  rise  to 
a  column  of  cells  standing  in  relation  to  the  white  rami. 
At  first  there  is  ao  indication  of  a  segmental  arrangement 


THE   SYMPATHETIC    SYSTEM. 


445 


of  the  cells  of  the  column  (Fig.  235),  but  at  about  the 
seventh  week  such  an  arrangement  makes  its  appearance 
in  the  cervical  region,  and  later,  extends  posteriorly,  until 
the  column  assumes  the  form  of  the  ganglionated  cord. 
Before,  however,  the  segmentation  becomes  marked, 


'  1 


Fig.  234. — Transverse  Section  throlcjh  the  Spinal  Cord  of  an  Em- 

?J    VO  OF  7   MM. 

c,  Notochord ;  g,  posterior  root  ganglion ;  m,  spinal  cord ;  s,  sympathetic 
cell  migrating  from  the  posterior  root  ganglion ;  wr,  white  ramus. — 
(His.) 


ii 


'  ■  H 


;  1;; 
f'l 


thickenings  appear  at  certain  regions  of  the  cell  column, 
and  from  these,  bundles  of  fibers  may  be  seen  extending 
ventrally  toward  the  viscera.  The  thickenings  represent 
certain  of  the  prae vertebral  ganglia,  and  later  cells  wander 
out  from  them  and  take  a  position  in  front  of  the  aorta. 


•  <i 


446 


THE    DEVELOPMENT   OF     i  IlK    HUMAN    ItODY. 


In  an  embryo  of  10.2  mm.  two  cfanglionic  masses  (Fijj. 
235,  pc)  occur  in  the  vicinity  of  the  origin  of  the  omphalo- 
mesenteric artery  {am),  one  lying  above  and  the  other 
below  that  vessel ;  these  masses  represent  the  ganglia  of 
the  solar  plexus  and  have  separated  somewhat  from  the 
ganglionated  cord,  the  fiber  bundles  which  unite  the  upper 
mass  with  the  cord  representing  the  greater  and  lesser 
splanchnic  nerves  {sp),  while  that  connected  with  the 
lowermass  represents  the  connection  of  the  cord  with  the 
superior  mesenteric  ganglion.  Lower  down,  in  the  neigh- 
borhood of  the  umbilical  arteries,  is  another  enlargement 
of  the  cord  {hg),  which  probably  represents  the  inferior 
mesenteric  and  hypogastric  ganglia  which  have  not  yet 
separated  from  the  cell  column. 

In  the  cervical  region  a  similar  origin  of  the  ganglion 
cells  of  the  cardiac  plexus  from  the  cell  column  seems  to 
obtain.  In  embryos  of  about  7  mm.  fibers  may  be  seen  ex- 
tending from  the  column  toward  the  heart,  and,  entering 
into  close  relationship  with  descending  branches  from  the 
vagus,  they  form  a  plexus,  the  ganglia  of  which  arc  com- 
posed of  cells  which  have  wandered  from  the  cell  column. 

The  elongated  courses  of  the  cardiac  sympathetic  and 
splanchnic  nerves  in  the  adult  receive  an  explanation  from  the 
recession  of  the  heart  and  diaphragm  (see  p.  259  and  342),  the 
latter  process  forcing  downward  the  solar  plexus,  which  origi- 
nally occupied  a  position  opposite  the  region  of  the  ganglio- 
nated cord  from  which  the  splanchnic  nerves  arise. 

The  cells  which  occur  in  the  peripheral  plexuses  have, 
in  a  similar  manner,  wandered  out  from  their  original 
position  in  the  cell  column.  In  10  mm.  embryos  groups 
of  such  cells  have  been  observed  both  in  the  lesser  and 
greater  curvatures  of  the  stomach  (Fig.  235,  *),  where 
they  become  connected  with  a  plexus  formed  by  fibers 
from  the  vagus  nerves  {rv).     The  wandering  of   sympa- 


THE  SYMPATHETIC   SYSTEM. 


447 


thetic  cells  into  the  walls  of  the  intestine  has  also  been 
observed,  and  they  form  at  first  a  single  la>er  in  the  meso- 
derm of  the  intestinal  wall,  only  later,  on  the  differentia- 
tion of  the  muscle  layers,  becoming  separated  into  the  two 


Fig.  235. — Reconstruction  of  the  Symp.\thetic  System  op  an  Em- 
bryo OF  10.2  MM. 

am,  Oinphalo-mesenteric  vein;  ao,  aorta;  au,  umbilical  artery;  bg,  gan- 
glionic mass  representing  the  pelvic  plexus ;  d,  intestine ;  oe,  oesopha- 
gus ;  pc,  ganglia  of  the  cceliac  plexus ;  ph,  pharynx ;  rv,  right  vagus 
nerve;  sp,  splanchnic  nerves;  sy,  ganglionated  cord;  t,  trachea;  *,  per- 
ipheral sympathetic  ganglia  in  the  walls  of  the  stomach. — {His,  Jr.) 


•  il 


J 


f 


1 


448  THE    DEVELOPMENT   OF   THE    HUMAN    BODY. 

layers  which  constitute  the  plexuses  of  Auerbach  and 
Meissner.  Similarly  cells  reach  the  heart  by  wandering  in 
some  cases  along  fibers  of  the  vagus,  although  they  really 
come  from  the  cervical  region  of  the  ganglionated  cord, 
and  having  in  their  wandering  met  with  fibers  of  the 
vagus,  make  use  of  them  as  paths  by  which  they  may 
reach  their  destination. 

As  regards  the  cephalic  sympathetic  ganglia,  the  ob- 
servations of  Remak  on  the  chick  and  Kolliker  on  the 
rabbit  show  that  the  ciliary,  sphenopalatine,  and  otic 
ganglia  arise  by  the  separation  of  cells  from  the  Gassenan 
ganglion,  and  from  their  adult  relations  it  may  be  sup- 
posed that  the  cells  of  the  submaxillary  and  sublingual 
ganglia  have  similarly  arisen  from  the  geniculate  ganglion 
of  the  facial  nerve.     Evidence  has  also  been  obtained  from 
human  embryos  that  sympathetic  cells  are  derived  from 
the  ganglia  of  the  vagus  and  glossopharyngeal  nerves,  but, 
instead  of  forming  distinct  ganglia  in  the  adult,  these,  in 
all  probability,  associate  themselves  with  the  first  cervical 
ganglia  of  the  ganglionas  •.  J  cord. 

Accessory  Organs  of  the  Sympathetic  System. -In  addi- 
tion to  the  various  sets  of  ganglia  which  clearly  belong  to 
the  sympathetic  svstem,  there  occur  throughout  the  body, 
in  various  regions,  certain  peculiar  organs  which  are 
closely  associated  with  the  same  system  both  in  their 
origin  and  in  their  adult  relations,  but  whose  exact  phys- 
iological significance  is  as  yet  problematical. 

The  Ganglia  Inter  car  otica. -These  structures,  which  are 
frequently  though  incorrectly  termed  carotid  glands  are 
small  bodies  about  5  mm.  in  length,  which  he  usually  to 
the  mesial  side  of  the  upper  ends  of  the  common  carotid 
arteries  Thev  possess  a  very  rich  arterial  supply  and 
stand  in  intimate  relation  with  the  branches  of  an  inter- 


ACCESSORY   SYMPATHETIC   ORGANS. 


449 


carotid  sympathetic  plexus,  and,  furthermore,  they  are 
characterized  by  possessing  as  their  specific  constituents 
markedly  cbromaffine  cells  (see  p.  392),  amonj,'  which  are 
scattered  stellate  cells  resembling  the  cells  of  the  sympa- 
thetic ganglia. 

They  have  been  found  to  arise  in  pig  embryos  of  44  mm. 
by  the  separation  of  cells  from  the  ganglionic  masses  scat- 


Fio.  236.— Section-  of  .\  Ceul  Ball  from  the  I.vtercarotid  Ganglion 

OF  Man. 

he    BUkkI    capillaries;    cv,    efferent   vein;    S,    ccmnective-tissue    sepUun ; 

/,  trahccuhe.— (/Vom  Bulim  and  Daridof},  ajhr  Schaper.) 

tcrcd  throughout  the  intercarotid  sympathetic  plexuses. 
Tlicse  cells,  which  become  the  chroma ffine  cells,  arrange 
themselves  in  round  masses  termed  cell  balls,  many  of 
which  unite  to  form  each  ganglion,  and  in  man  each  cell 
ball  becomes  broken  up  into  trabeculae  by  the  blood- 
vessels (Fig.  236)  which  penetrate  its  substance,  and  the 
38 


It   ■" 
K 


I. 


1 


450 


-HE    DKVKI.Ol'MKNT    OK     THK    HUMAN    IIODY. 


individual  halls  arc  separated  from  one  another  hy  con- 
siderahle  (iiiantities  of  connective  tissue. 

.Some  confusion  has  existed  in  the  past  as  to  the  origin  of  this 
structure.     The  mesial  wall  of  the  proximal  part  of  the  internal 
carotid  artery  becomes  ccmsiderably  thickened  (hiring  the  early 
stages  of  development  and  the  thickening  is  traversed  by  nunier 
ous  Vdood  lacuna'  which  communicate  with  the  lumin  of  the 
vessel.     This  condition  is  perhaps  a  relic  of  the  branchial  capil 
laries  which  in  the  lower  gill-breathing  vertebrates  rejiresent 
the  proximal  i)ortion  of  the  internal  carotid  and  has  nothing  to 
do  with  the  formation  of  the  intercarotid  ganglion,  although  it 
has  beiii  believed  bv  some  authors  (Schai)er)  that  the  ganglion 
was  derived  from  the  thickening  of  the  wall  of  the  vessel.      The 
fact  that  in  some  animals,  such  as  the  rat  and  the  dog,  the  gaii 
glion  stands  in  relation  with  the  external  carotid  and  receivts 
its  blood  supply  from  that  vessel  is  of  importance  in  this  con 

nection.  ,     ,  •  , 

The  thickening  of  the  internal  carotid  disappears  m  the  higher 
vertebrates  almost  entirely,  but  in  the  Amphibia  it  persists 
throughout  life,  the  lumen  of  the  proximal  part  of  the  vessel 
being  converted  into  a  line  meshwork  by  the  numerous  tra- 
becula-  which  traverse  it.  This  carotid  labyrinth  has  been 
termed  the  carotid  gland,  a  circumstance  which  has  probably 
assisted  in  producing  confusion  as  to  the  real  significance  of  the 
intercarotid  .^^anglion. 

The  (^rqa  s  of  Zuckcrkandl.— In  embryos  of  14.5  mm. 
there  have  been  found,  in  front  of  the  abdominal  aorta, 
closely  packed  groups  of  cells  which  resemble  in  appear- 
ance the  cells  composing  the  ganglionated  cord,  two  of 
these  groups,  which  extend  downward  along  the  side  of  the 
aorta  to  below  the  point  of  origin  of  tlie  inferior  mesen- 
teric artery,  being  especially  distinct.  These  cell  groups 
give  rise  to  the  ganglia  of  the  prevertebral  sympathetic 
plexuses  and  also  to  peculiar  bodies  which,  from  their 
discoverer,  may  be  termed  the  organs  of  z^uckerkandl. 
Kach  body  stands  in  intimate  relaticm  with  the  fibers  of 
the  sympathetic  plexuses  and  has  a  lUli  blood-supply, 


ACCESSORY    SYMI'MHKIIC    OKCANS. 


45  > 


rcscniblinjj  in  these  respects  the  intenarotid  jianijHa,  and 
the  reseinbhiiice  is  further  increased  by  the  fact  that  the 
specilic  cells  of  tl     or^'an  are  markedly  ehrotnafhne. 


n.r. 


i.r.s. 


Fig.  2.n.— Accessory  Svmi-atiiktic    okcans   ov  Z' cki:rk.\m>i,  kkom 

A  Xsw-born;  Ciiilii 
a,  Aorta;  ci,  infiTior  vein  cav.i ;  /.c,  tciiniium  iliac  artc-ry;   »",   inferitir 

mesftitcric    aru-ry;    >t.l  and    'i.r,   \v(t   ami   riulit   actossury   i>rj;ans ; 

f>l.a,  aortic  pk-xus;  »,  urttii  ,  ;./.<.  left  rcn  il  vein,    -{/.tickirkandl.) 

At  birth  the  bodies  situated  in  the  upi^er  portion  of  the 
abdominal  cavity  have  broken  up  into  small  masses,  but 
the  two  lower  ones,  mentioned  above,  are  still  well  defined 
(Fig.  237).     liven  these,  however,  seem  to  disappear  later 


::  r 


Ml 


452 


THE  I)KVEi.oi'Mi:nt  of  the  human  noDV. 


on  and  no  traces  of  them  have  as  yet  been  fount,  in  the 

adult. 

The  Coccyqiiil  or  Luxchka's  Gtniglw)t.—h\  embryos  of 
about  I  .s  cm.  there  is  to  be  found  on  the  ventral  snrf-ice  of 
the  apex  of  the  coccyx  a  small  oval  group  "^  polygonal 
cells,  clearly  separated  from  the  surrounding  tissue  by  a 
mesenchymal  capsule.  Later,  connective-tissue  trabe- 
cular make  their  way  into  the  masN,  which  thus  becomes 
divided  into  lobules,  and,  at  the  same  time,  a  rich  vascu- 
lar supply,  derived  principally  from  brandies  of  the 
arteria  sacra  media,  penetrates  the  body  which  thus  as- 
sumes the  adult  condition,  in  which  it  presents  a  general 
resemblance  to  the  intercarotid  ganglion. 

There  are  as  vet  no  direct  observations  determining  the 
origin  of  the  specific  cells  of  this  coccygeal  gland,  but  the 
evidence  available  points  to  their  derivation  from  the 
sympathetic  system.  They  appear  in  the  position  which 
should  be  occupied  by  the  terminal  portion  of  the  sympa- 
thetic cord,  and  from  the  time  when  they  first  become 
recognizable  onward  they  are  connected  with  sympathetic 
fibers.  The  probability  is  that,  Hke  the  cells  of  the  otlier 
organs  described  above,  they  are  derived  from  sympa- 
thetic ganglia. 


LITERATURE. 

W.  His;  "  Zur  Gescliichte  des  mcnscliliclien  Ruckenmarkes  und  dcr  Ner- 
venwurzclii,"  Ahliatidl.  dcr  konigl.  Sdclisisclun  GcscUsch..  Matli.- 
Physik.  Classf,  xilt,  1886. 

W.  His:  "Zur  Geschichtc  des  (iehirns  sowie  der  centralen  und  peripher- 
ischen  Nervenbahnen  beiin  iiienschlichen  Kmbryo,"  AbhanJl.  der 
konif^l.  Sdchsischen  Ge.scllsch.,  Mitih.-i'lt)sik.  Class>-,  Xiv,  1888. 

\V.  His:  "Die  Torment wickehing  des  menschlichen  Vorderliirns  vom 
Ende  des  ersten  bis  zum  Beginn  des  dritten  Mimats,"  Alihandl.  der 
komgl.  Siichsischcn  Gcselhch.,  Maih.-Phy.nk.  Classe,  xv,  1889. 

W.  His:  "  Histogenese  und  Zusarnnienhang  der  Nerveneleniente,"  Archir 
fur  Anat.  und  Physiol.,  Anat.  Ahth.,  Supplement,  1890. 


LITlCkATLRE. 


453 


w 


Hi«»,   JR.:    "Die    I'ntvxi   Wilun«    'les   Ht- 


rzntr\  tnsvstetns  1  vi    U'irVwl 


thieren, 

Clam-   XViii,  IH'Ai 


\hliuiill    'l>r    A:""'k'      ^.i./(w.<7;.im, 


Kcll  .  M  ■!'>    '''H  't' 


\V   His 


|r  .      Teberdic  Kntw»ckelvi!n;<U  '  i«,ii    lis> 


hicii''  hfim  Hulm 


cli*-n    iinil    MensfliPii,"    \r,l,ii   jur  Aii.it      <«./    .  'i .  <i'<i 

Su'hl.»ir,it.   lf<'»7.  ...... 

C    I    HiJRklCK:  "ii      Cranial  :i '1(1  First '^i.inaiXcrvts.t  Menuha      \  u.n 

'  ■   ;ribu.i<.n  u|K.n  ^m■  Nirxc  L  .mip'-uctn  ,  "f  tin    H-ny  I  islu  s"  ./■  ""'   "/ 
*  ,,.h       Wiirol.,  IX,  IH''^ 
C    1     Hekkick     "Tl'.e  Cranial  Ni-rvc- ..n.l  Cuta.u-..u>  S.  nsi;-...  ^ans  m  tlie 

■  N    rth  AnuT-can  Silti.oi.,  i-islas."   /      n,   .>H\->nf    \.urol.,  XI.  l'"»l, 
r.    C.    h    BKK        l""nr   I.ectnus  ,,,1  tin-  SviiM-atli.    ic   Nervous  Systt-in." 

l,>uin    i'/i'i)m/>.  .V.MMi/..     II,  IH')T. 
I.    n,    (.XKoBssoN-      •HeitTiu.   /nr  kc-nnu.       <Ur  fo    .U-n  KnlwickU.ng  dtr 

Slfissdrust'."  .1m/(/;  /iir  mikr.nk.  Aiiat.,  i.iii,  !SW. 
A.   Koll^.  "reherden  Han  una  dii-  i:n.«ickflun«  der  s..k.  CarntisdrUse,' 

\rchn    inr  niikro.k    .\mil.,  LVI,  \')iiO. 
M     VON    1.  :Mi()ssEK        Die   l-ntwu  U-luii>!     .^r  (Vanglien  .nlasen  bei  dein 
nienschlidien  Hinl.ryn."   Anlm    jur  A>:.it.  u,„l  rhyuoL.    Amtt.  Ahlh.. 

IS'Jl.  ...   , 

I.-.  Marchani.    "UehtT  dk    Untwickeluug  dis   Halkcns  iin   inenscl.licl.cti 

Gehirn      .In/i-i/Mr  mikroslc.  A>i,it  .  xxxvii,  18'M. 
V.  VON'  Mi     NUK..VRZ    "liMtwickelunKSKeschichte  des  Gehirns,"  Leipzig, 

1877 
A.    D.    Oyuui:  "rebtrdi.-  I'tilwickeluiiK  des  sympatlnschcn  Nervrnsys- 

tems,"  .Irr/nr  /i<r  mikio^k.  A  tint  ,  XXVII.  1«S(). 
G    RETZiLS.  "  Da     Miiischcnliirn,"  Stockholm,  IH'>6 
A  Schaher:  "  DielruliesU-nDitTerenzirunKsvor-an-e  im  Cintraln.rvin 

system."  Ariliii  ju>  luilu  icklnih^'onrrhauik.  V,  18')7. 
().  S.  STRON.i;     ■•  The  vranial    .Wrvc-s  of     Aniphil.ia,"    Journal    oj    Mor 

/>/!.)/. ,  X,  189.S. 
R.  \Vi..-:^sak:   "  Die  Fkrkiinll  dc,  Myelin^.,     Arclir,  jur  h»lunklun)!.sm,  - 

il^imk,  VI,   18'»8. 
K    Zuckerkandl:   ■•  rchtr  Nchfnors'an-desSy  npithicusim  Rttropn 
tonealraum  des  Menschen,'     V.rluimll.  Amit.  r/.  v, //.sd/.,  xv,  l')Ol. 


CHAFl'KR  XV. 


THE     DEVELOPMENT     OF    THE     ORGANS     OF 
SPECIAL    SENSE. 

Like  the  cells  of  the  central  nervous  system,  the  sensory 
cells  are  all  of  ectodermal  origin,  and  in  lower  animals, 
such  as  the  earthworm,  for  instance,  they  retain  their  orig- 
inal position  in  the  ectodermal  epithelium  throughout 
life.  In  the  vertebrates,  however,  the  majority  of  the  sen- 
sory cells  relinquish  their  superficial  position  and  sink 
more  or  less  deeply  into  the  subjacent  tissues,  being  repre- 
sented by  the  posterior  root  ganglion  cells  and  by  the  sen- 
sory cells  of  the  special  sense-organs,  and  it  is  only  in  the 
olfactory  organ  that  the  original  condition  is  retained. 
Those  cells  which  have  withdrawn  from  the  surface  re- 
ceive stimuli  only  tlirougli  an  overlying  cell  or  cells,  and  in 
certain  cases  these  transmitting  cells  are  not  specially 
differentiated,  the  terminal  branches  of  the  sensory  den- 
drites ending  among  ordinary  epithelial  cells  or  in  such 
structures  as  tlie  Pacinian  bodies  or  the  end-bulbs  of 
Krause  situated  beneath  undifferentiated  epithelium. 
In  other  cases,  however,  certain  specially  modified  super- 
ficial cells  serve  to  transmit  the  stimuli  to  the  peripheral 
sensory  neurones,  forming  such  structures  as  the  hair-cells 
of  the  auditory  epithelium  or  of  the  taste-buds. 

Thus  three  degrees  of  differentiation  of  the  special  sen- 
sory cells  may  be  recognized  and  a  classification  of  the 
sense-organs  ma\  be  made  upon  this  basis.  One  organ, 
however,  the  eye,  cannot  l)e  brought  into  such  a  classifica- 
tion, since  its  sensory  cells  present  certain  developmental 

454 


THE   OLFACTORY    NERVE. 


455 


peculiarities  which  distinguish  them  from  those  of  all  other 
sense-organs.     Hmbryologically  the  retina  is  a  portion  of  ^ 
the  central  nervous  system  and  not  a  peripheral  organ,  ' 
and  hence  it  will  be  convenient  to  arrange  the  other  sense- 
organs  according  to  the  classification  indicated  and  to 
discuss  the  history  of  the  eye  at  the  close  of  the  chapter. 

The  Development  of  the  Olfactory  Organ.— The  general 
development  of  the  nasal  fossa,  the  epithelium  of  which 
contains  the  olfactory  sense  cells,  has  already  been  de- 
scribed (pp.  97  and  104),  as  has  also  the  development  of 
the  olfactory  lobes  of  the  brain  (p.  427).  and  it  remains  to 
consider  here  merely  the  formation  of  the  olfactory  nerve 
and  the  development  of  the  rudimentary  organ  of  Jacob- 


son. 


The  Olfactory  Nerve.— Very  diverse  result:,  have  been 
obtained  by  various  o'oservcrs  of  the  development  of  the 
olfactory  nerve,  it  having  been  held  at  different  times  that 
it  was  formed  by  the  outgrowth  of  fibers  from  the  olfac- 
tory lobes  (Marshall),  from  fibers  which  arise  partly  from 
the  olfactory  lobes  and  partly  from  the  olfactory  epithe- 
lium (Beard),  from  the  cells  of  an  olfactory  ganglion  origi- 
nally derived  from  the  olfactory  epithelium  but  later 
separating  from  it  (His),  and,  finally,  that  it  was  composed 
of  the  prolongations  of  certain  cells  situated  and,  for  the 
most  part  at  least,  remaining  permanently  in  the  olfactory 
epithelium  ( Disse).     The  most  recent  observations  on  the 
structure  of  the  olfactory  epithelium  and  nerve  indicate 
a  greater  amount  of  probability  in  the  last  result  than  in 
the  others,  and  the  description  which  follows  will  be  based 
upon  the  observations  of  His,  modified  in  conformity  with 
the  results  obtained  by  Disse  from  chick  embryos. 

In  human  embryo  f  the  fourth  week  the  cells  lining 
the  upper  part  of  the  olfactory  pits  show  a  distinction  into 
ordinary  epithelial  and  sensory  cells,  the  latter,  when  fully 


. 


456 


THE    DEVELOPMENT    OF    THE    HUMAN    I50DV. 


formed,  being  elongated  cells  prolonged  peripherally  into 
a  short  but  narrow  process  which  reaches  the  surface 
of  the  epithelium  and  proximally  gives  rise  to  an  axis- 
cylinder  process  which  extends  up  toward  and  penetrates 


s: 
I 


I 

i 

! 


I 


1! 


I'k;.    2,^8.       I)l.\(,K.\M    lLUtSTKATfN<-.    THK    RELATIONS    OK    THU    I'iBKKS    OF 

THE  olkactokv  \i:k\e. 

Hf>,  Epitlielium  of  the  olfactory  pit ;  (  ,  ciiliriforiu  iilate  of  ttic  ct!iiii<>i(l ; 

G,  glomerulus  ol  the  olfactory  l)ulh;  ."t/,  mitral  cell. — {Wiii  (iilituiilm.) 


the  tip  of  the  olfactory  lobe  to  come  into  contact  with  the 
dendrites  of  the  hrsl  central  neurones  of  the  olfactory 
tract  (Fig.  2,^8).  These  cells  constitute  a  neuro-epitheliuni 
and  in  later  stages  of  development  retain  tlieir  epitlieliul 


tA^J. 


THE    OROAN    OF    JACOBSON. 


457 


position  for  the  most  part,  a  few  of  tljem,  however,  with- 
ctrawin^  into  the  subjacent  mesenchyme  and  becomini; 
bipolar,  their  peripheral  i)rolonKations  ending  freely 
among  the  cells  of  the  olfactory  epithelium.  These  hi 
polar  cells  resemble  closely  in  form  and  relations  the  cells 
of  the  embrvonic  posterior  root  g^niKl'^i.  and  thus  form  an 
interesting  transition  between  these  and  the  neuro-epithe- 

lial  cells. 

The  Orqan  of  Jacohson.— In  embryos  of  three  or  four 
months  a  small  pouch-like  invagination  of  the  epithelium 
covering  the  lower  anterior  portion  of  the  median  septum 
of  the  nose  can  readily  be  seen.  This  becomes  converted 
into  a  slender  pouch,  3  to  5  mm.  long,  ending  l)lindly  at  its 
posterior  extremity  and  opening  at  its  other  end  into  the 
nasal  cavitv.  Its  lining  epithelium  resembles  that  of  the 
respiratorv  portion  of  tlie  nasal  cavity,  and  there  is  devel 
oped  in  the  connective  tissue  beneath  its  floor  a  slender 
plate  of  cartilage,  distinct  from  that  forming  the  septum 

of  the  nose. 

This  organ,  which  may  apparently  undergo  degcnera 
tion  in  the  adult,  and  in  some  cases  completely  disap- 
pears, appears  to  be  the  representative  of  what  is  known  as 
Jacobson's  organ,  a  structure  which  reaches  a  much  more 
extensive  degree  of  development  in  many  of  the  lower 
mammals,  and  in  these  contains  in  its  epithelium  sensory 
cells  whose  axis-cylinder  processes  pass  with  those  of  the 
olfactory  sense  cells  to  the  olfactory  bulbs.     In  m;in,  how- 
ever, it  seems  to  be  a  rudimentary  organ,  and  no  satisfac- 
tory explanation  of  its  function  has  as  yet  been  advanced. 
The    olfactory    neuro-epitlielium.    considered    from    a 
comparative  standpoint,  seems  to  have  been  derived  from 
the  system  of  lateral  line  organs  so  liiglily  developed  \n  the 
lower  vertebrates.      In  higher  forms  the  system,  which  is 
cutaneous  in  character,  has  disappeared  except  in  two 


K'-vuM'p-^-i'iie- 


»'^-*i/amnm>ff 


458 


TTTE    DFAKI.OPMENT    OF   THE    HUMAN    BODV. 


I   I 


regions  where  it  has  become  highly  specialized.  In  one  of 
these  regions  it  has  given  rise  to  the  olfactory  sense  cells 
and  in  the  other  to  the  similar  cells  of  the  auditory  appara- 
tus. 

The  Organs  of  Touch  and  Taste.-  Nothing  is  yet  known 
concerning  the  development  of  the  various  forms  of  tactile 
organs,  which  belong  to  the  second  class  of  sensory  organs 
described  above. 

The  Organs  of  Taste.  —The  remaining  organs  of  special 
sense  belong  to  the  third  class,  and  of  these  the  organs  of 
taste  present  in  many  respects  the  simplest  condition. 
They  are  developed  principally  in  connection  with  the 


ryTt 


Fir,.  2.V),  -Di.\<;r.\ms  Rkpkese.vti.nm  the   DivViii.op.ME.NT  of  .\  Circu.m- 

V.XLLATE  P.\I'II.I,.\. 

a,  Valley  surroundinj^  the  ])apilla;  h,  von  IJl)ner's  jjland. — {Crahrg.) 

circumvallate  and  foliate  papilhr  of  the  tongue,  and  of  the 
former  one  of  the  earliest  observed  stages  has  been  found 
in  embryos  of  9  cm.  in  the  form  of  two  ridges  of  epidermis, 
lying  toward  the  back  part  of  the  tongue  and  inclined  to 
one  another  in  such  a  manner  as  to  form  a  V  witii  the  apex 
directed  backward.  From  these  ridges  solid  down- 
growths  of  epidermis  into  the  subjacent  tissue  occur,  each 
downgrowlh  having  the  form  of  a  hollow  truncated  cone 
with  its  basal  edge  continuous  with  the  superficial  epider- 
mis i'F"ig.  2,^9,  A).  In  later  stages  lateral  outgrowths  de- 
velop from  the  deeper  edges  of  the  cone,  and  about  the 
same  time  clefts  appear  in  the  substance  of  the  original 


THE  ORGANS  OF  TASTE. 


459 


downgrowths  (Fig.  239,  B)  and,  uniting  together,  finally 
opt'n  to  the  surface,  forming  a  trench  surrounding  a  pa- 
pilla (Fig.  239,  C).  The  lateral  outgrowths,  which  are  at 
first  solid,  also  undergo  an  axial  degeneration  and  become 
converted  into  the  nlands  of  Elmer  (h),  which  open  into 
the  trench  near  its  floor.  The  various  papilla?  which  occur 
in  the  adult  do  not  develop  simultaneously,  but  their  num- 
ber increases  with  the  age  of  the  fetus,  and  there  is,  more- 
over, considerable  variation  in  the  time  of  their  develop- 
ment. 

The  ta&te-buds  are  formed  by  a  differentiation  of  the 
epithelium  which  covers  the  papilla-,  and  this  differentia- 
tion appears  to  stand  in  intimate  relation  with  the  pene- 
tration of  fibers  of  the  glossopharyngeal  nerve  into  the  pa- 
pilla-. The  buds  form  at  various  places  upon  the  papilla-, 
and  at  one  period  are  especially  abundant  upon  their  free 
surfaces,  but  in  the  later  weeks  of  intrauterine  life  these 
surface  buds  undergo  degeneration  and  only  those  upon 
the  sides  of  the  trench  persist,  as  a  rule. 

The  foliate  papilla-  do  not  seem  to  be  developed  until 
some  time  after  the  circum vallate,  being  entirely  wanting 
in  embryos  of  four  and  a  half  and  five  months,  although 
plainlv  recognizable  at  the  seventh  month. 

The  Development  of  the  Ear.— It  is  customary  to  de- 
scribe the  mammalian  car  as  consisting  of  three  parts, 
known  as  the  inner,  middle,  and  outer  ears,  and  this  divi- 
sion is,  to  a  certain  extent  at  least,  confirmed  by  the  em- 
bryonic development.  The  inner  ear,  which  is  the  sen- 
sorv  portion  proper.  Is  fundamentally  an  ectodermal 
structure,  secondarily  becoming  deeply  seated  in  the 
mesodermal  tissue  of  the  head,  while  the  middle  and  outer 
cars,  which  provide  the  apparatus  necessary  for  the  con- 
duction of  the  sound-waves  to  the  inner  ear,  are  modified 
portions  of  the  anterior  branchial  arches.     It  will  be  con- 


^m 


n"Wi 


i 

fl     ' 


460 


THE    nEVELOPMENT    OF    THE    HUMAN    BODY. 


venieiit,  accordinijly,  in  the  description  of  the  ear,  to  ac- 
cept the  usually  recojjnized  divisions  and  to  consider  first 
of  all  the  development  of  the  inner  ear,  or,  as  it  is  better 
termed,  the  otocyst. 

The  Development  oj  the  (Uocysi. — In  an  embryo  of  2.4 
mm.  a  ])air  of  pits  occur  upon  the  surface  of  the  body 
about  opposite  the  middle  portion  of  the  hind-brain  (Fig. 
240,  A).  The  ectoderm  lining  the  pits  is  somewhat 
thicker  than  is  the  neighl)oring  ectoflerm  of  the  surface 
of  the  body,  and,  from  analogy  with  what  occurs  in  other 
vertebrates,  it  seems  probable  that  the  pits  are  formed  by 
the  invagination  of  localized  thickenings  of  the  ectoderm. 


Ki. 


240.      Ik  \\>\  i;ksi-;   Skciiun    Passim,    thkoich   tiik   (Jtocyst   (ot) 
in-    j:MBk\()S  oi-   (.1)   2.4  MM.   .\M>  (li)   4  MM.-    (His.) 


The  nioutli  of  each  pit  gradually  becomes  smaller,  until 
finally  the  invaginatirm  is  converted  into  a  closed  sac  (Pig. 
240.  H  ,  which  separates  from  the  surface  ectoderm  and 
becomes  enclosed  within  the  subjacent  mesoderm.  This 
sac  is  the  ot<x.\vst,  and  in  the  stage  just  described,  found 
in  eml^ryos  of  4  mm.,  it  has  an  oval  or  more  or  less  spheri- 
cal form.  Soon,  howe\er,  in  eml)ryos  of  6.9  mm.,  a  pro- 
longation arises  front  its  dorsal  portion  and  the  sac  as- 
suffies  the  form  shown  in  iMg.  241,  A;  this  prolongation 
rt-|>Tesents  tlie  ductus  endolyniphaticus,  and,  increasing  in 
length,  it  soon  becomes  a  strong  club  shaped  process,  pro- 
jecting considerably  bexond  the  remaining  portions  of  the 


1 


THE    INTEKN.M.    FAR. 


461 


otocyst  (Fig-  241,  B).  In  embryos  of  about  10.2  mm. 
the  sac  bct,nns  to  show  certain  other  irrej^uUirities  of  shape 
(Fig.  24 1 ,  B,  sc).  Thus,  about  opposite  the  point  of  origin 
of  the  ductus  enclolym{)haticus  three  fohls  make  their 
appearance,  representing  the  semicircular  auuils,  and  as 


— \sc 


Fig   241  -Reconstr.ctions  oi-  the  Otocvsts  of  Ivmbkvos  of  (.1)  6.9 

MM     AND   (li)    10.2  MM. 

dc    Fnclolvniphatic  duct;   ur,  >;anslinn  cocl.k-arc:  j;g,  ^i'-.'^ili"" /™'^'- 
tunT    fir,  sanKlinn  vestilmlar.;    sc,  hc>n/...ntal  scnncircular  canal - 

(Ills',  Jr.) 

they  increase  in  size  the  opposite  walls  of  the  central  por- 
tion of  each  fold  come  together,  fuse,  and  finally  become 
absorbed,  leaving  the  free  edge  of  the  fold  as  a  crescentic 
canal,  at  one  end  of  which  an  enlargement  appears  to 
form  the  ampulhi.  The  transformation  of  the  folds  into 
canals  takes  place  somewhat  earlier  in  the  cases  of  the  two 


I  !' 


■jp? 


^^EBW 


^mmm 


mm 


11 


462 


THE    UEVEI.OPMENT   OF   THE    HUMAN    ItOOY. 


vertical  than  in  that  of  the  horizontal  canal,  as  may  be 
seen  from  Fig.  242,  which  represents  the  condition  oc- 
currinj;  in  an  embryo  of  13.5  mm. 

A  short  distance  below  the  level  at  which  the  canals 

communicate  with  the  re- 
maining portion  of  the  oto- 
cyst  a  constriction  appears, 
indicating  a  separation  of  the 
otocyst  into  a  more  dorsal 
portion,  which  becomes  the 
utriculus,  and  a  more  ventral 
one.  Later,  the  ventral  por- 
tion of  the  latter  begins  to  be 
prolonged  into  a  flattened 
canal  which,  as  it  elongates, 
becomes  coiled  upon  itself 
and  also  becomes  separated 
by  a  constriction  from  the 
portion  of  the  otocyst  from 
which  it  arises.  The  latter  is 
the  representative  of  tiie 
adult  sacculus  Fig.  243,  s), 
while  the  coiled  canal  (co) 
forms  the  scala  media  of  the 
cochlea  and  the  constricted 
portion  of  the  otocyst,  which 
unites  the  scala  and  the  sac- 
culus, becomes  the  canalis 
rcunicns.  The  constriction 
i  which  marks  the  line  of  sep- 

aration of  tlie  utriculus  '«/)  and  sacculus  is  converted 
into  a  narrow  canal  witli  which  the  ductus  endolympli- 
aticus  connects,  and  hence  it  is  that,  in  the  adult,  the 
cDnnectinn  between  these  two  portions  of    the  otocyst 


Fig.  242. — Keconstrlction  of 

THE    OTOCVST  (IF  .\N   I{mBRVU 
OF    1,V5    MM. 

CO,  Cochlea;  t/f,  fnddlymjiliafic 
du  " ;  sc,  semicircular  canal, 
-(//'v,  jr.) 


THE    INTERNAL    KAK. 


463 


seems  to  be  formed  by  the  ductus  dividing  proximally 
into  two  limbs,  one  of  which  is  connected  with  the  utri- 
cle and  the  other  with  the  saccule. 

When  first  observed  in  the  human  embryo  the  auditory 
ganglion  is  closely  associated  with  the  geniculate  ganglion 
of  the  seventh  nerve  (Fig.  241,  B),  the  two,  usually 
spoken  of  as  the  acustico-facialis  ganglion,  forming  a  mass 


Fk;.    2■^^.  -RecONSTRI-CTION  OF    THK  <  )TOCYST  OK   AN    KmBRYO  OK  22    MM. 

CO,  Cochlea:  ,k,  endolyniphalic  duct.v,  sacculus;  ul,  utriculus.-  (//n,  Jr.) 

of  cells  h  ing  in  close  contact  ^vith  the  anterior  wall  of  the 
otocyst.  The  origin  of  the  ganglionic  mass  lias  not  yet 
been  t.aced  in  the  mammalia,  but  it  has  been  observed 
thai  in  cow  em'oryos  the  genieulatt  ganglion  is  connected 
with  ihe  ectoderm  at  the  dorsal  en  1  of  the  first  branchial 
cleft  (Froriep),  and  it  may  perhaps  be  regarded  as  one  of 


I  JWtU' 


i 


I 
I 


464 


THE    DEVELOPMENT    OF     THE    HUMAN    IH)PV. 


the  cpihrancliial  j:;aiiKlia  (sec  p.  440),  and  in  the  lower 
vertebrates  a  union  of  the  ganglion  with  a  suprabranchial 
ganglion  has  been  observed  (kupfer),  this  union  indicat- 
ing the  origin  of  the  auditory  ganglion  from  one  or  more 
of  the  ganglia  of  the  lateral  line  system. 

At  an  early  stage  in  the  human  emSryo  the  auditory 
ganglion  shows  indications  of  a  division  into  two  portions, 
a  more  dorsal  one,  which  represents  the  future  (jamjlion 
vestihularc,  and  a  ventral  one,  the  ganglion  cochleare.  The 
ganglion  cells  become  bipolar,  in  which  condition  they 
remain  throughout  life,  never  reaching  the  T-shajied 
condition  found  in  most  of  theotl"  r  peripheral  cerebro- 
spinal ganglia.  One  of  the  prolongations  of  each  cell  is 
directed  centrally  to  form  a  fiber  of  the  auditory  nerve 
while  the  other  penetrates  the  wall  of  the  otocyst  to  enter 
into  relations  with  certain  specially  modified  cells  which 
dilTcrentiate  from  its  lining  epithelium. 

In  the  earliest  stages  the  ectodermal  lining  of  the  oto- 
cyst is  formed  of  similar  columnar  cells,  but  later  over  the 
greater  part  of  the  surface  the  cells  flatten  down,  only  a 
few,  aggregated  together  to  form  i)atches,  rcl .fining  the 
high  colunmar  form  and  developing  hair-like  procp'^es 
upon  their  free  surfaces.  These  are  the  sensory  ..^I  of 
the  ear.  In  the  human  ear  there  are  in  all  six  patches  of 
these  sensory  cells,  an  elongated  patch  (crista  acusiica) 
in  the  ampulla  of  each  semicircular  canal  (P'ig.  244,  cr),a 
round  patch  (macula  acustica,  mu)  in  the  utriculus  and 
anotlicr  (ms)  in  the  sacculus,  and,  finally,  an  elongated 
patch  v\hicli  extends  the  entire  length  of  the  scala  media 
of  the  cochlea  and  forms  the  sensory  cells  of  the  organ  of 
Corti. 

In  connection  with  this  last  patch  certain  adjacent  cells 
also  retain  their  columnar  form  and  undergo  various  modi- 
fications, giving  rise  to  a  rather  complicated  structure 


il 


Till     INTKRNAL    EAR. 


465 


whose  development  has  been  traced  in  the  rabbit.  AloiiK 
the  whole  length  of  the  seala  media  the  cells  resting  upon 
that  half  of  the  basilar  membrane  which  is  nearest  the  a.xis 
of  tlK>  cochlea,  and  may  be  termed  the  inner  half,  retain 


r/f 


Pif,    244-  The  RuiiiT  Intervm.   Hak  "P  an  Hmbkyo  ok  vSix  Months 
ca    cc,  and   cb.    Anterior,  [external,  and    jx-sterior    setnicirciilar    canals; 
cr  .rista  acustica ;  i/r,  endolviiiiiluitic  duct;  /v,  spiral   liKanient  ;    mh, 
basilar  mcmhrane;   ms   and    w»,  macula   acustica    sacculi   and    utn- 
culi;  W',  basilar  branches  of  the  cochlear  nerve.     (Rdznts.) 

their   columnar    shape,    forming    two    ridges   projecting 
slightly  into   the  cavity  of  the  scala    (Fig.    245).     The 
cells  of  the  inner  ridge,  much  the  larger  of  the  two,  give 
rise  to  the  mcmhrana  tcctona,  cither  as  a  cuticular  sccre 
39 


'  ml 

I     Sit  I 

!  i  ■ 


1  I: 


I   ii 


. 


L!  i  F 


1.0 


I.I 


Ik 

ISO 


2.8 

■  40 


2.5 
2.2 

2.0 


1.8 


MICROCOPY  RESOLUTION  TEST  CHART 

NATIONAL  BUREAU  OF  STANDARDS 

STANDARD  REFERENCE  MATERIAL  1010a 

(ANSI  and  ISO  TEST  CHART  No.  2) 


466 


THK    DKVELOr.MEM"    OF    THE    HUMAN    BODV. 


tion  or  by  the  artificial  adhesion  of  long  hair-like  processes 
which  project  from  their  free  surfaces  (Avers).  The  cells 
of  the  outer  ridge  are  arranged  in  six  longitudinal  rows 
(Fig.  245,  1-6);  those  of  the  innermost  row  (i)  develop 
hairs  upon  their  free  surfaces  and  form  the  inner  hair  cells, 
those  of  the  next  two  rows  (2  and  t,)  gradually  become 
transformed  on  their  adjacent  surfaces  into  chitinous  sub- 


Fin.  24,S.  -SiXTinx  or  tiik  Scaua  Miciha  of  thk  Cochlea  ok  a  Rabbit 

I{mbk\o  of  5.1  MM. 

(J,    .Mcstnchyim';   /.  tn  , ,  t"j)itlK'liuni   of   si ;i la  media  ;   .1/./,  iiieiiilirana  tec- 

turia;  r.*^./>,  vein;  I  tn  7,  nrj^an  i>f  Corti. — {liiigiud'y.) 


Stance  and  form  the  rods  of  Corti,  while  the  three  outer 
rows  (4  to  6)  develop  into  the  outer  hair  cells.  It  is  in 
connection  with  the  hair  cells  that  the  peripheral  pro- 
longations of  the  cells  of  tlie  cochlear  ganglion  terminate, 
and  since  these  hair  cells  are  arranged  in  rows  extending 
the  entire  length  of  the  scala  media,  the  ganglion  also  is 


TIIK    INTEKNAI.    KAR. 


4h 


drawn  out  into  a  spi 


ral  followinir  tlic  coils  of  tlic  coclika, 


and  lience  is  soinctinK's  termed  the  s]  iral  t,fan,i,dion. 
While  tlie  variou> 


IS  ci 


hanees  described  above  have  bein 


takint;  place  in   the  olocyst,  the  mesoderm  surroundm.i; 
it  has  also  been  underi^ointj  development.     At  first  this 
tissue  is  quite  uniform  in  character,  but  later  the  cells 
immediatt  'v  surroundini,^  the  otocyst  condense  to  t,dve  rise 
to  a  fibrous  layer  fFi,l,^  246,  cp)  while  more  peripherally 
they  become  more  loosely  arraniijed  and  form  a  some- 
what ,t,a-latinous  layer  is), 
anci  still  more  peripher- 
dlly     a     second     fibrous 
layer     is     dilTerentiated 
and  the  remainder  of  the 
tissue  assumes  a  charac- 
ter   which    indicates    an 
approaching    conversion 
into  cartilage.     The  fur- 
ther history  of  these  va- 
rious layers  is  as  follows. 
The   inner  fibrous   layer 
gives  rise  to  the  connect- 
ive-tissue wall  which  sup- 
ports the  ectodermal  lin- 
ing of    the  various  por- 
tions of  the  otocyst;    the  gelatinous  layer  undergoes  a 
degeneration    to   form   a  lymph-like  fluid  known  as  the 
perilvmph,  the  space  occupied  by  the  fluid  being  the  peri- 
lymphatic space;  the  outer  fibrous  layer  becomes  peri- 
chondrium and  later  periosteum;    and  the  procartilage 
undergoes  chondrification  and  later  ossifies  to  form  the 
petrous  portion  of  the  temporal  bone. 

The  gelatinous  la3'cr  completely  surrounds  most  of  the 
otocyst  structures,  which  thus  come  to  lie  free  in  the  peri- 


P  - 
»"- 

ej> 


Imi;.       246.  —  TkansvicksK      Sixtion 

THROUGH     .\     .SivMICIKClXAK     C.WAU 

OK  A  Rabbit  1vmbkv(j  oi"  Twkntv- 
FouR  Days. 
c,  Periolic  cartilage;  (7>,  fihrmis  inciii 
hrane  beneath  the  epithelium  nf  tlie 
canal;  {^,  perichnndriiiiii ;   ^,  sp<ii\i;y 
tissue. — (\'<»!  Kollik-rr.) 


111 


468 


TIIK    DEVEI.Or.MKNT    OF    THK    HUMAN    BOOV, 


lymphatic  space,  but  in  the  cochlear  rej^ion  the  conditions 
are  somewhat  different.  In  this  region  clie  gelatinous 
layer  is  interrupted  along  two  lines,  an  outer  broad  one 
where  the  connective-tissue  wall  of  the  scala  media  is 
directly  continuous  with  the  perichondrium  layer,  and  an 
inner  narrow  one,  along  which  a  similar  fusion  takes  place 
with  the  perichondrium  of  a  shelf-like  process  of  the  car- 
tilage, which  later  ossifies  to  form  tlie  lamina  spiralis. 


Fi(..  247.  -  DiAc.KAMMATic  Tra.nsvkrse  Section-  throuc.ii  a  Coil  of  the 

Cochlea,  showint.  the  Relations  or  the  Sc\L/K. 

r,  Organ  i)f  Corti;  co,  jjuni^lion  cochleare;  Is,  lamina  spiralis;  SM,  scala 

media;  .S'7",  scala  lyinpani;  .ST,  scala   vestihnli. — {From  Gcrhich.) 

Consequently  throughout  the  cochlear  region  the  peri- 
lymphatic space  is  divided  into  two  compartments  which 
communicate  at  the  ape.x  of  the  cochlea,  while  below^  one, 
known  as  the  scala  vcstibuli,  comnmnicates  with  the  space 
surrounding  the  saccule  and  utricle,  and  the  other,  the 
scala  tymparti,  ahuts  upon  a  membrane  which  separates  it 
from  the  cavity  of  the  middle  ear  and  represents  a  portion 


THE    MIIMII.K    KAK. 


469 


of  the  outer  wall  of  the  petrous  bone  where  chondriticatioii 
and  ossification  have  failed  to  occur.  This  membrane 
closes  what  appears  in  the  dried  skull  to  be  an  opening  in 
the  inner  wall  of  the  middle  ear,  known  from  its  shape  as 
the  fenestra  rotunda;  another  similar  opening,  also  closed 
by  membrane  in  the  fresh  skull,  occurs  in  the  bony  wall 
opposite  the  utricular  portion  of  the  otocyst  and  is  known 
as  the  fenestra  oralis. 

The  Development  0}  the  Middle  £(/r.— The  middle  car 
develops  from  the  upper  part  of  the  pharyngeal  groove 
which  represents  the  cndodermal  portion  of  the  lirst 
branchial  cleft.  This  becomes  prolonged  dorsally  and  at 
its  dorsal  end  enlarges  to  form  the  tympanic  cavity,  while 
the  narrower  portion  intervening  between  this  and  the 
pharyngeal  cavity  represents  the  Ivustachian  tube. 

To  correctly  understand  the  development  of  the  tym- 
panic cavity  it  is  necessary  to  recall  the  stj  actures  which 
form  its  boundaries.  Anteriorly  to  the  upper  end  of  the 
first  branchial  pouch  there  i-  the  upper  end  of  the  first 
arch,  and  behind  it  the  corresponding  part  of  the  second 
arch,  the  two  fusing  together  dorsal  to  the  tympanic 
cavity  and  forming  its  roof.  Internally  the  cavity  is 
bounded  by  the  outer  wall  of  the  cartilaginous  investment 
of  the  otocyst,  wliile  externally  it  is  separated  from  the 
upper  part  of  the  ectodermal  groove  of  the  first  branchial 
cleft  by  the  thin  membrane  which  forms  the  floor  of  the 

groove. 

It  has  b.en  seen  in  an  earlier  chapter  that  the  axial 
mesoderm  of  each  branchial  arch  gives  rise  to  skeletal 
structures  and  muscles.  The  axial  cartilage  of  the  ventral 
portion  of  the  first  arch  is  wlat  is  known  as  Meckel's 
cartilage,  but  in  that  portion  of  the  arch  which  forms  the 
roof  and  anterior  wall  of  the  tympanic  c  '-ty,  the  car- 
tilage becomes  constricted  to  form  two  .nasses  wliicii  later 


i  '4 


470 


TIIK    DKVEr.OI'MK.vr    OF     IIIK    HUMAN    I'.ODY. 


ossify  to  form  the  UKillcus  and  incus  (Vi^.  248,  m  and  ;'), 
uliile  tlie  muscular  tissue  of  tliis  dorsal  j)ortion  of  the  arch 
,<,Mves  rise  to  the  linsot  tymfxnii.  Similarly,  in  the  case  c!" 
the  second  arch  there  is  to  be  found,  dorsal  to  the  extrem- 
ity of  the  cartila,i(e  which  forms  the  styloid  process  of  the 
adult,  a  narrow  plate  of  cartilaj;e  which  forms  an  invest- 
ment for  the  facial  nerve  (Fii;-.  248,  17/),  and  dorsal  to 


ri<;.  248,— Semi  DiA  .rammatic  \'ii;\v  ok  tiii-  ArDiTOKV  Ossicuks  ok  an 

I{MBKyo  OF  Six  Wkkks. 
I,  Incus;  y,  jugular  vein;  m,  malleus;  mc,  Meckel's  cartilage;  or,  cajjstile 

of  (itocyst;  A',  cartilaj;c  of  the  second  branchial  arcii ;  st,  stapes;  VII, 

facial  ncTYe.~—(Sul)cnmaun.) 


1-t 


this  a  rini,r  of  cartilajre  {st}  which  surrounds,  a  small  artery 
and  represents  the  stapes,  in  connection  with  which  a 
muscle,  termed  the  stapedius,  develops. 

Since,  as  has  already  been  stated,  the  two  arches  meet 
dorsally  above  the  primitive  tympanic  cavity,  the  struc- 
tures just  mentioned  lie  embedded  in  the  mesenchyme 
forming  the  roof  of  the  cavity,  as  does  also  the  chorda 


Pi 


TIIK    MIDDI.K    K..R.  47  I 

tympani,  a  braticli  of  the  seventh  nerve,  us  it  passes  into 
tiie  substance  of  the  first  arch  on  the  way  to  its  destina- 
tion. The  mesenchyme  in  which  these  various  structures 
are  embedded  is  rather  voluminors  {V'v^.  250).  and  after 

the  end  of  the  seventh  month  

it  becomes  converted   into  a      ''  ' 

peculiar  spongy  tissue,  wliich,  ^      ^-v 

tov^^ard  the  end  of   fetal  life,  /■        .-    "•, 

gradually     degenerates,     the  '••.  ,.' 

tympanic  cavity  at  the  same  

time  expanding  and  wrapping     ^     _,   .  .       ..  „. 

itself  around  the  ossicles  and  .-•  "      K         "  \ 

the  muscles  attached  to  them  ''^J^ 

(Fig.   249).      The   bones   and         "  \  j 

their   muscles,    consequently,  •.,.         r 

while  appearing  in  the  aduU  ■"  ^ 

to    traverse     the     tym.panic     ^ ,.,>x»w»»«»>--..„p.,^. .,-.^^  .„-,:..„ 

cavity,  are  really  completely        m  ,-''       ,^.. 
enclosed  within  a  layer  of  epi-           ■'           ''"•■ 
thelium  continuous  with  that            \             ^ 
lining  the  wall  of  the  cavity,  '•••.. .'"'      ^ 

while  the  handle  of  tlie  mal- 

,    ,,         11.                 •  Fk;      249.     Dia(1k.\ms     iU.rs- 

leus  and  the  chorda  tympani  tratinm;  thk  Mope  ok  Kx 

lie  between  tl:e  epithelium  of  tension  ok  the  Tympanic 

^                   .  Cavity  Akoind  the    Aiui- 

the  outer  vvall  of  the  cavity  tory  ossicueh, 

and     the      fibrous      mesoderm       M,  Malleus;  m,  Ki).;ngy  mcscn 

cliviiie;  />,  Hirer  s-irface  ut  tlic 
which     forms     the     tympanic  pc'riotic  capKule;  7,  tymranic 

mr.mKt-nnc.  caviiv.     Tlic  hnikoti  line  rej)- 

membrane.  ^^.^^^^^  ^j,^  epithelial  lining  of 

The  extension  of  the  tym-         iiie  lympanic  cavity, 
panic   cavity  does   not,  how- 
ever, cease  with  its  replacement  of  the  degenerated  spongy 
mesenchyme,  but  toward  the  end  of  fetal  life  it  begins  to 
invade  the  substance  of  the  temporal  bone  by  a  process 
similar  to  that  which  produces  the  ethmoidal  cells  and  the 


P 


47 


TllK    DEVKI.OPMENT    OF     TIIK    HUMAN    HODY. 


laii 


-I 


otlicr  osseous  sinuses  in  connectioti  with  the  nasal  caviti( 
(see  p.  i9(;).  This  process  continues  for  some  year^  aft< 
birth  and  results  in  the  formation  in  the  mastoid  purtio 
of  the  bone  of  the  so-called  mastoiiJ  cells,  which  commun 
cate  with  the  tympanic  cavity  and  have  an  epitheli: 
lining  continuous  with  that  of  the  cavity. 

The  lower  portion  of  the  diverticulum  from  the  fin 
pharyngeal  groove  which  gives  rise  to  the  tympanic  cavil 
becomes  converted  into  the  Ivustachian  tube.  Uurin 
development  the  lumen  of  the  tube  disappears  for  a  tim< 
probpbly  owing  to  a  proliferation  of  its  lining  epitheliun 
but  it  is  re-established  before  birtli. 

In  the  account  of  the  developtnenl  of  the  ear-bones  give 
above  it  is  held  that  the  malleus  and  incus  are  derivatives  of  tl: 
first  branchial  (mandibularj  arch  and  the  stapes  of  the  secont 
This  view  rejjresenls  the  general  consensus  of  recent  workers  o 
the  difficult  question  of  tlie  origin  of  these  bones,  but  it  shoul 
be  mentioned  that  nearly  all  possible  modes  of  origin  have  bee 
at  one  time  or  other  suggested.  The  malleus  has  very  generall 
been  accepted  as  coming  from  the  first  arch,  and  the  same 
true  of  the  incus,  although  some  earlier  authors  have  assigne 
it  to  the  second  arch.  But  with  regard  to  the  stapes  the  opii 
ions  have  been  very  varied.  It  has  been  held  to  be  derive 
from  the  first  arch,  from  the  second  arch,  from  neither  one  no 
the  other,  but  from  the  cartilaginous  investment  of  the  otocys 
or,  finally,  it  has  been  held  to  have  a  compound  origin,  its  arc 
being  a  product  of  the  second  ireh  while  its  basal  plate  was 
part  of  the  otocyst  investment.  Recent  observations  seem  t 
pliice  its  independence  of  the  otocyst  investment  beyon 
doubt,  in  which  case  its  origin  from  the  second  arch  seen 
fairly  certain. 

The  Development  of  the  Tympanic  Membrane  and  of  th 
Outer  Ear.-  Just  as  the  tympanic  cavity  is  formed  fror 
the  endodermal  groove  of  the  first  branchial  cleft,  so  th 
outer  ear  owes  its  origin  to  the  ectodermal  groove  of  th 
same  cleft  and  to  the  neighboring  arches.  The  dorsi 
and  most  ventral  portions  of  the  groove  flatten  out  am 


Tiir.  i;.\TEi<N\r.  ink. 


473 


1  cavities 
•ar^  after 
1  portion 
onimuiii- 
?pithelial 

the  first 

lie  cavity 

During 

r  a  time, 

ithelium, 


ncs  give, 
ves  of  the 
le  second, 
•orkers  on 
it  should 
lave  been 
generally 
e  same  is 
'  assigned 
>  the  opin- 
e  derived 
ir  one  nor 
e  otocyst, 
1.  its  arch 
ate  was  a 
s  seem  to 
I  beyond 
rch  seems 


ud  of  the 
led  from 
t,  so  the 
:q  of  the 
e  dorsal 
out  and 


disappear,  Imt  the  median  portion  deepens  to  form  at 
alumt  the  end  of  the  sec(md  month,  a  finmel  shaped 
cavity  which  corresponds  to  the  onter  portion  of  the  ex- 
ternal aiulitorv  meatus.     I'rom  the  itmer  end  «»f  this  a 


Fig.  250.  -Horizontal  Section  I'.\ssin«;  thkoich  tiiic  Dorsal  Waui, 

OF  THE    H.VTERNAl.   AuDlTOKV    MEATI'S   IN   A.V    I{mKKVO   Ol*   4.5    CM. 

c,  Cochlea;  dc,  endolyiniihatic  duct;  i,  incus;  Is,  lateral  sinus;  m,  malleus; 
me,  meatus  auditorius  cxternus;  mc',  cavity  of  tlic  meatus;  s,  sac- 
culus;  sc,  horizontal  semicircular  canal;  sc',  jjosterior  semicircular 
canal;  si,  stapes;  /,  tympanic  cavity;  u.  itriculus;  7,  facial  nerve. — 
{Sichenmann.) 

solid  ingrowth  of  ectoderm  takes  place,  and  this,  enlari,Mn>,^ 
at  its  inner  end  to  form  a  disk-like  mass,  comes  into  rela- 
tion with  the  st-"lathious  mesoderm  which  surmmids  the 
malleus  and  chorda  tympani.  At  about  the  seventh 
40 


■i  \ 


474 


THE    UEVELOI'MKNT    OF    THE    HUMAN    IIODY. 


u 


month  a  spHt  occurs  in  the  disk  like  mass  (I'va-  250), 
separating  it  into  an  outer  and  an  inner  hiyer,  the  latter  of 
which  becomes  the  outer  epithelium  of  the  tympanic 
membrane.  I^ater,  the  ^plit  extends  outward  in  the  sub- 
stance of  the  ectodermal  injjrowth  and  eventually  unites 
with  the  funnel-shaped  cavity  to  (omplete  the  external 
meatus. 

The  tympanic  membrane  is  formed  in  considerable  part 
from  the  substance  of  the  first  !)ranchial  arch,  the  area  in 
which  it  occurs  not  being  primarily  part  of  the  wall  of  the 
tympanic  cavity,  but  being  brought  into  it  secondarily 
by  the  expansion  of  the  cavity.  The  membu  ?  itself  is 
mesodermal  in  origin  and  is  lined  on  its  outer  surface  liy 
an  ectodermal  and  on  the  inner  by  an  cndodermal  epi- 
thelium. 

The  pinna  owes  its  origin  to  the  portions  of  the  first  and 
second  arches  which  bound  the  entrance  of  the  external 
meatus.  Upon  the  posterior  edge  of  the  first  arch  there 
appear  about  the  end  of  the  fourth  week  two  transverse 
furrows  which  mark  off  three  tubercles  (Fig.  251,  A,  1-3) 
and  on  the  anterior  edge  of  the  second  arch  a  correspond- 
ing number  of  tubercles  (4-6)  is  formed,  while,  in  addition, 
a  longitudinal  furrow,  running  down  the  middle  of  the 
arch,  marks  off  a  ridge  (c)  lying  posterior  to  the  tubercles. 
Troni  these  six  tubercles  and  the  ridge  are  developed  the 
various  parts  of  t.ie  pinna,  as  may  be  seen  from  Fig.  251. 
The  most  ventral  tubercle  of  the  first  arch  (i )  gives  rise  to 
the  tragus,  and  the  middle  one  (5)  of  the  second  arch  fur- 
nishes the  antitragus.  The  middle  and  dorsal  tubercles  of 
the  first  arch  {2  and  3)  u.iite  with  the  ridge  (c)  to  produce 
the  helix,  while  from  the  dorsal  tubercle  of  the  second  arch 
(4)  is  produced  the  antihelix  and  from  the  ventral  one  (6) 
the  i  ^'le.  It  is  noteworthy  that  at  about  the  third 
month  of  development  the  upper  and  posterior  portion  of 


rilK    1  XlKRWr.    KAK. 


475 


the  IkHx  is  IkiU  forward  so  as  to  conceal  the  antihelix; 
is  at  just  about  a  correspoiulinj;  staj^e  that  the  pointetl 
form  of  the  ear  ^een  in  tlie  lower  tnanuuals  makes  its 
appearance,  and  it  is  evident  that,  were  it  not  for  the  for- 
ward hendin,l,^  the  human  ear  would  also  be  assuming'  at 


Fk;.  251.     St.v.ks  in  tiiE  Deveuopment  of  the  Pinna. 
A,  Ivinhi y<>  "f  1 1  iiitii. ;  H,  of  13.6  mm. ;  C,  of  15  mm. ;  I),  at  the  beginning 
'  of  the  third  month;  A",  fetus  of  8.5  cm.;  /■",  fetus  at  term.— (//i,?.) 

th.is  sta^e  a  more  or  less  pointed  form.  Indeed,  there  is 
usually  to  be  found  upon  the  incurved  edge  of  the  helix, 
some  distance  below  the  upper  border  of  the  pinna,  a  more 
or  less  distinct  tubercle,  known  as  Datwin-A  tubercle,  which 
seems  to  rep-  .ent  the  point  of  the  typical  mammalian 


. « 1 

i 


■  \  S"! 


^ 

_    w^ 

1 1       B 

ki 

II 

■)  ■ 

ill 

m 


it 


m 


■I'i 


476 


IIK     I)K\  I  lOI'MKNT    Of    TIIK    IH  MAN    IIODV. 


car,  ami  is,  accordinj^ly,  tlu-  inorpholoi^ical  ajjcx  of  the 
pinna. 

There  seems  to  h;  httle  room  for  (loiil)t  t!iat  the  otocyst  he 
loiiRS  i)rimarily  to  tlie  system  of  hiteral  hue  sense-organs,  hut  a 
(hscussion  t,f  this  interesting  cjuestion  would  neeessilate  a  con 
sideration  of  details  concerning  the  development  of  the  lower 
vertehrates  which  would  he  foreign  to  the  general  i)lan  ol  this 
l,>ook.  It  may  he  recalled,  however,  that  the  analysis  of  the 
comi)ouents  of  the  cranial  nerves  descrihed  on  ])age  437  refers 
the  auditory  nerve  to  the  lateral  line  system. 

The  Development  of  tht  Eye.- The  first  indications  of 
the  development  of  the  eye  are  to  be  found  in  a  pair  of 
hollow  outgrowths  from  the  side  of  the  first  primary  brain 
vesicle,  at  a  level  which  corresponds  to  the  junction  of  the 
dorsal  and  ventral  zones.  Ivacli  evagination  is  directed  at 
first  upward  and  backward,  and,  enlarging  at  its  extrem- 
ity, it  soon  '.hr^  s  a  differentiation  into  a  terminal  bulb 
and  a  s\'all.  coiuiecting  the  bulb  with  the  brain  (Fig.  216). 
At  an  early  stage  the  bulb  comes  into  apposition  with  the 
ectoderm  of  the  side  of  the  head,  and  this,  over  the  area 
of  contact,  becomes  thickened  and  then  depressed  to  form 
the  beginning  of  the  future  lens  (Fig.  252). 

As  the  result  of  the  depression  of  the  lens  ectoderm,  the 
outer  wall  of  the  optic  bulb  becomes  pushed  inward  to- 
ward the  inner  wall,  and  this  invagination  conti\ming  until 
the  two  walls  come  into  contact,  the  bulb  is  transformed 
into  a  double-walled  cup,  the  optic  cup,  in  the  mouth  of 
which  lies  the  lens  (Fig.  254).  The  cup  is  not  of^rfect, 
however,  since  the  invagination  affects  not  only  tiie  optic 
bulb,  but  also  extends  inward  on  the  posterior  surface  of 
the  stalk,  forming  upon  this  a  longitudinal  groove  and  pro- 
du  ^  a  defect  of  the  ventral  wall  of  the  cup,  known  as 
the  chorioidal  tissure  (Fig.  253).  The  groove  and  fissure 
become  occupied  by  mesodermal  tissue,  and  in  this,  at 


illK    r.VK. 


477 


ahoul  llu-  fifth  wvvk,  a  I )1<m)(1- vessel  (Kvilop-  lidi  tra- 
verse the  cavity  of  the  eup  tt)  reach  the  lens  and  is  known 
as  tht  artcria  hvnloulca. 


Fir..  252.— Karly  Stale  in  the  Develop.ment  of  the  Lens  in  a  Rabbit 

liMBRYO. 

The  nucleated  layer  i.i  the  left  is  the  ectoderm  and  the  thicker  lens 
epithelium,  below  which  "-  the  outer  wall  of  the  optic  evagination; 
above  and  below  between  the  two  is  mesenchyme. — {Rabl.) 

In  the  mean  time  further  changes  have  been  takingplace 
in  the  lens.     The  ectodennal  depression  which  represents 


t 


1 


478 


THE    DEVELOPMENT   OF   THE    HUMAN    nOOY. 


it  gradually  deepens  to  form  a  cup,  the  lips  of  which  ap- 
proximate and  finally  meet,  so  that  the  cup  is  converted 
into  a  vesicle  which  finally  separates  completely  from  the 
ectoderm  (Fig.  254),  much  in  the  same  way  as  the  otocyst 
does.  As  the  lens  vesicle  is  constricted  off,  the  surround- 
ing mesodermal  tissue  grows  in  to  form  a  layer  between 
it  and  the  overlying  ectoderm,  and  a  split  appearing  in  the 
layer  divides  it  into  an  outer  thicker  portion,  wliich  rep- 
resents the  cornea,  and  an  inner  thinner  portion,  which 


Fio.  253. — Reconstruction  of  the  Brain  of  an   Embryo  of  Toir 
Weeks,  showinc.  the  Chorioid  Fissure. — (His.) 

covers  the  outer  surface  of  the  lens  and  becomes  highly 
vascular.  The  cavity  between  these  two  portions  repre- 
sents the  anterior  chamber  of  the  eye.  The  cavity  of  the 
optic  cup  has  also  become  filled  by  a  peculiar  tissue  which 
represents  the  vitreous  humor,  while  the  mesodermal  tissue 
surrounding  the  cup  condenses  to  form  a  strong  inv^est- 
ment  for  it,  which  is  externally  continuous  with  the 
cornea,  and  at  about  the  sixth  week  sliows  a  differentia- 
tion into  an  inner  vascular  layer,  the  chorioid  coat,  and  an 
outer  denser  one,  which  becomes  the  sclerotic  coat. 


THE    LENS. 


479 


The  various  processes  resulting  in  the  formation  of  the 
eye,  which  have  thus  been  rapidly  sketched,  may  now  be 
considered  in  greater  detail. 

The  Development  oj  the  Lens.— ^'\\cn  the  lens  vesicle  is 
complete,  it  forms  a  more  or  less  spherical  sac  lying  be- 


i  \ 


Fig.  254.— Horizont.\l  Section  through  the  Eve  ok  an  E.mbryo  Pig 

OF  7  MM. 

lir,  Thalamencephalon ;  Ec,  ectoderm;  /,  lens;  P,  pigment,  and  R,  retinal 

layers  of  the  retina. 

ncath  the  superficial  ectoderm  and  containing  in  its  cavity 
a  few  cells,  either  scattered  or  in  groups  (Fig.  254).  These 
cells,  which  have  wandered  into  the  cavity  of  the  vesicle 
from  its  walls,  take  no  part  in  the  further  development  of 
the  lens,  but  early  undergo  complete  degeneration,  and 


48o 


THK    nUVELOPMKNT   OF     THK    HUMAN    I'.ODV. 


the  first  change  which  is  concerned  with  the  actual  forma- 
tion of  the  lens  is  an  increase  in  the  height  of  the  cells 
forming  its  inner  wall  and  a  thinning  out  of  its  outer  wall 
(Fig.  255,  A).  These  changes  continuing,  the  outer  half 
of  the  vesicle  becomes  converted  into  a  single  layer  of 
somewhat  flat  cells  which  persist  in  the  adult  condition  to 
form  the  anterior  epithelium  of  the  lens,  while  the  cells  of 
the  posterior  wall  form  a  marked  projection  into  the  cav- 
ity of  the  vesicle  and  eventually  completely  obliterate  it, 
coming  into  contact  with  the  inner  surface  of  the  anterior 
epithelium  (Fig.  255,  B). 

These  posterior  elongated  cells  form,  then,  the  principal 
mass  of  the  lens,  and  constitute  what  are  known  as  the 
lens  fibers.  At  first  those  situated  at  the  center  of  the  pos- 
terior wall  are  the  longest,  the  more  peripheral  ones  gradu- 
ally diminishing  in  length  until  at  the  equator  of  the  lens 
they  become  continuous  with  and  pass  into  the  anterior 
epithelium.  As  the  lens  increases  in  si.  ?,  however,  the 
most  centrally  situated  cells  fail  to  elongate  as  rapidly  as 
the  more  peripheral  ones  and  are  pushed  in  toward  the 
center  of  the  lens,  the  more  peripheral  fibers  meeting  be- 
low them  along  a  line  passing  across  the  inner  surface  of 
the  lens. '  The  disparity  of  growth  continuing,  a  similar 
sutural  line  appears  in  the  outer  surface  beneath  the  ante- 
rior epithelium,  and  the  hbcrs  become  arranged  in  con- 
centric layers  around  a  central  core  composed  of  the 
shorter  fibers.  In  ll.e  human  eye  the  line  of  suture  of  the 
peripheral  fibers  becomes  bent  so  as  to  consist  of  two  limbs 
which  meet  at  an  angle,  and  from  the  angle  a  new  suturing 
line  develops  during  embryonic  life,  so  that  the  suture 
assumes  the  form  of  a  three-rayed  star.  In  later  life  the 
stars  become  more  complicated,  being  either  six-rayed 
or  more  usually  nine-rayed  in  the  adult  condition  (Fig. 
256). 


Pig    25S— Suctions  through  me  Lens  (.4)  of  Human  Embryo  of 
'  THmTY  TO  Thirtv-onk  Davs  and  (U)  of  Pig  Embryo  of  36  mm.— 


\l 


i    f 


k 


I 


}  I 


482 


THE    nF.VEI,OPMENT    OF   THE    HUMAN    BODY. 


As  early  as  the  second  month  of  development  the  lens 
ve^'irle  becomes  completely  inverted  by  mesodermal  tissue 
in  which  blood-vessels  are  developed  in  considerable  num- 
bers, whence  the  investment  is  termed  the  tunica  vascu- 


FiG.  256.— PosTEKioR  (Inner)  Sikkace  of  the  Len's  from  an  Adult 

SHOWINC   THE  SlTURAL  Ll.N'ES.— (A'oW.) 


losa  lentis  (Fig.  264,  tv).  7"  e  arteries  of  th^  cunic  are  in 
connection  principally  with  the  hyaloid  artery  of  the 
vitreous  humor  (Fig.  262),  and  consist  of  numerous  fine 
branches  which  envelop  the  lens  and  terminate  in  loops 


II 


THE    (tl'TIC    CUP. 


483 


almost  at  the  center  of  its  outer  surface.  This  tunic  un- 
dergoes degc.eration  after  the  seventh  month  of  develop- 
ment, •  which  time  the  1  s  has  completed  its  period  of 
most  active  growth,  and,  a.  a  rule,  completely  disappears 
before  birth.  Occasionally,  however,  it  may  per  ist  to  a 
greater  or  less  extent,  the  persistence  of  the  portion  cover- 
ing the  outer  surface  of  the  lens,  known  as  the  niembrana 
pupiUaris,  causing  the  malformation  known  as  congemial 

atresia  oj  the  pupil.  ^ 

In  addition  to  the  vascular  tunic,  the  lens  is  surrounced 
by  a  non-cellular  membrane  termed  the  capsule.  The 
origin  of  this  structure  is  still  in  doubt,  some  observers 
maintaining  that  it  is  a  product  of  the  investing  meso- 
derm, while  others  hold  it  to  be  a  product  of  the  kus  epi- 
thelium. 

The  Development  of  the  Optic  Cup.-Whcn  the  invagina- 
tion of  ine  outer  wall  of  the  optic  bulb  is  completed,  the 
margins  of  the  resulting  cup  are  opposite  the  sides  of  the 
lens  vesicle  (Fig.  254),  but  with  the  enlargemen      '  the 
lens  and  cup  the  margins  of  the  latter  gradually  c     -le  to 
lie  in  front  of— that  is  to  say,  upon  the  outer  surface  of— 
tlie  lens,  forming  the  boundary  of  the  opening  known  as 
the   pupil.     The  lens,   consequently,   is  brought   to  lie 
within  the  mouth  of  the  optic  cup,  and  th?t  portion  of  the 
latter  which  covers  the  lens  takes  part  in  the  formation  of 
the  iris  and  the  adjacent  ciliary  body,  while  its  posterior 
portion  gives  rise  to  the  retina. 

The  chorioidal  fissure  normally  disappears  during  the 
sixth  or  seventh  week  of  development  by  a  fusion  of  its 
lips,  and  not  until  this  is  accomplished  does  the  term  cup 
truly  describe  the  form  assumed  by  the  optic  bulb  after 
the  invagination  of  its  outer  wall.  In  certain  cases  the 
lips  of  the  fissure  fail  to  unite  perfectly,  producing  the  de- 
fect of  the  eye  known  as  colohoma;  this  may  vary  in  its 


i 

■  i 


484 


THE    DEVELOPMENT    OF    THE    HUMAN    UOPV, 


extent,  sometimes  affecting  both  the  iris  and  the  retina 
and  forming  what  is  termed  colobonia  iridis,  and  at  others 
being  confined  to  the  retinal  po'*ion  of  the  cup,  in  which 
case  it  is  termed  coloboma  chorioideae. 

Up  to  a  certain  stage  the  differentiation  of  the  two 
layers  which  form  the  optic  cup  proceed.-,  along  si»^iilar 
lines,  in  both  the  ciliary  and  retinal  regions.  That  layer 
which  represents  the  original  internal  portion  of  the  bulb 
becomes  thinner  as  the  cup  increases  in  size,  and  becomes 
also  the  seat  of  a  deposition  of  dark  pigment,  whence  it 
may  be  termed  the  pigment  layer  of  the  cup;  while  the 
other  layer — that  formed  by  the  invagination  of  the  outer 
portion  of  the  bulb,  and  which  may  be  termed  the  retinal 
layer — remains  much  thicker  (Fig.  254)  and  in  its  proxi- 
mal portions  even  increases  in  thickness.  Later,  however 
the  development  of  the  ciliary  and  retinal  portions  of  the 
retinal  la}  ers  differs,  and  it  will  be  convenient  to  consider 
first  the  history  of  the  ciliary  portion. 

The  Development  of  the  Iris  and  Ciliary  Body. — The  first 
change  noticeable  in  the  ciliary  portion  of  the  retinal  layer 
is  its  thinning  out,  a  process  which  continues  until  the 
layer  consists,  like  the  pigment  layer,  of  but  a  single  layer 
of  cells  (Fig.  257),  the  transition  of  which  to  the  thicker 
retinal  portion  of  the  layer  is  somewhat  abrupt  and  corre- 
sponds to  what  is  termed  the  orascrrata  in  adult  anatomy. 
In  embryos  of  10.2  cm.  the  retinal  layer  throughout  its 
entire  extent  is  readily  distinguishable  from  the  pig.nent 
layer  by  the  absence  in  it  of  all  pigmentation,  but  in  older 
forms  this  distinction  gradually  diminishes  in  the  iris  re- 
gion, the  retinal  layer  there  acquiring  pigment  and  form- 
ing the  tivea. 

When  the  anterior  chamber  of  the  eye  is  formed  by  the 
spUtting  of  the  mesoderm  which  has  grown  in  between  the 
superficial  ectoderm  and  the  outer  surface  of  the  lens,  the 


THE    IRIS    AND    CILIARY    IKDY, 


485 


peripheral  portions  of  its  posterior  (inner)  vail  are  in  rela- 
tion with  the  ciliary  portion  of  the  optic  cup  ami  give  rise 
to  the  stroma  of  the  ciliary  body  and  of  the  iris  (Fig.  257) 
this  latter  being  continuous  with  the  tunica  vn  culosa 
Icntis  so  long  as  that  structure  persists  (Fig.  264).  In 
embrvos  of  about  14.J  cm.  the  ciHary  portion  of  the  cup 


Fig  257  —Radial  Section  through  the  Iris  ok  an  Hmbryo  of 

19    CM. 

1£,  PiRment  kiyer;  CC,  ciliary  folds;  IE,  retinal  layer;  /..S7r  iris 
stroma;  /^m,  pupillary  membrane;  Rs,  marginal  sinus ,  .S/)/j,  sphinc- 
ter iridis.-(Szi/i.) 


becomes  thrown  into  radiating  folds  (Fig.  257),  as  if  by  a 
too  rapid  growth,  and  into  the  folds  lamellae  of  mesoderm 
project  from  the  stroma.  These  folds  occur  not  only 
throughout  the  region  of  the  ciUary  body,  but  also  extend 
into  the  iris  region,  where,  however,  they  are  but  tem- 
porary structures,  disappearing  entirely  by  the  end  of  the 


486 


THE    DEVELOPMENT   OF    THE    HUMAN    BOl>Y. 


\$ 


I 

N 


>  I 


fifth  month.     The  folds  in  the  region  of  the  corpus  ciliare 
persist  and  produce  the  ciliary  processes  of  the  adult  eye. 

Embedded  in  the  substance  of  the  iris  stroma  in  the 
adult  aij  non-3triped  muscle-fibers,  which  constitute  the 
sphincter  and  dilatator  iridis.  It  has  long  been  supposed 
that  these  fibers  were  difTerentiated  from  the  stroma  of  the 
iris,  but  recent  observations  have  shown  that  they  arise 
from  the  cells  of  the  pigment  layer  of  the  optic  cup,  the 
sphincter  appearing  near  the  pupillary  border  (Fig.  257, 
sph )  while  the  dilatator  is  more  peripheral. 

The  Development  of  the  Retina. — Throughout  the  retinal 
region  of  the  cup  the  pigment  layer,  undergoing  the  same 
clianges  as  in  the  ciliary  region  forms  the  pigment  laver 
of  the  retina  (Fig.  258,  p).  The  retinal  layer  increases 
in  thickness  and  early  becomes  differentiated  into  two 
strata  (Fig.  254),  a  thicker  one  lying  next  the  pigment 
layer  and  containing  numerous  nuclei,  and  a  thinner  one 
containing  no  nuclei.  The  thinner  layer,  from  its  position 
and  structure,  suggests  an  homology  with  the  marginal 
velum  of  the  central  nervous  system,  and  probably  be- 
comes converted  into  the  nerve-fiber  layer  of  the  adult 
retina,  the  axis-cylinder  processes  of  the  ganglion  cells 
passing  into  it  on  their  way  to  the  optic  nerve.  The 
thicker  layer  similarly  suggests  a  comparison  with  the 
mantle  layer  of  the  cord  and  brain,  and  in  embryos  of  38 
mm.  it  becomes  differentiated  into  two  secondary  layers 
(Fig.  258),  that  nearest  the  pigment  layer  [  consisting 
of  smaller  and  more  deeply  staining  nuclei,  probably 
representing  the  rod  and  cone  and  bipolar  cells  of  the  adult 
retina,  while  the  inner  layer,  that  nearest  the  marginal 
velum,  has  larger  nuclei  and  is  presumably  composed  of 
the  ganglion  cells. 

Little  is  as  yet  known  concerning  the  further  differentia- 
tion of  the  nervous  elements  of  the  human  retina,  but  the 


M 


THK    RKTINA. 


487 


history  of  some  of  tlicm  has  been  traced  in  the  cat,  in 
which,  as  in  other  mammals,  the  histoj,'enetic  processes 
take  place  at  a  relatively  later  period  than  in  man.  Of  the 
histogenesis  of  the  inner  layer  the  infor'uation  is  rather 
scant,  but  it  may  be  stated  that  the  ganglion  cells  are  the 
earliest  of  all  the  elements  of  the  retina  to  become  recog- 


00  o*^  O  O       O         O 

Fi(i.  258.— PoRTi  IN  OF  A  Transverse  .Section  ok  the  Retina  of  a 

New-born  Rabbit. 
ch,  Chorioid  coat;  g,  ganglinn-cell  layer;  r,  outer  layer  of  nuclei;  />,  pig- 
ment layer.— (/"(i/f/n.) 


nizable.  The  rod  and  cone  cells,  when  first  distinguish- 
able, are  unipolar  cells  fFig.  259,  a  and  c),  their  single 
processes  extending  outward  from  the  cell-bodies  to  the 
external  limiting  membrane  which  bounds  the  outer  sur- 
face of  the  retinal  layer.  Even  at  an  early  stage  the  cone 
cells  {a)  are  distinguishable  from  the  rod  cells  (c)  by  their 


488 


THE    DKVELOI'MENT    OF    THK    HUMAN    IIODV. 


V\ 


I 


H 


more  decided  reaction  to  silver  salts,  und  at  hrst  both 
kinds  of  cells  are  scattered  throuRhout  the  thickness  of 
the  layer  from  which  they  arise.  Later,  a  fine  process 
grows  out  from  the  inner  end  of  each  cell,  which  thus  as- 
sumes a  bipolar  form  (Imr.  259.  />  and  d),  and,  later  still, 
the  cells  gradually  migrate  toward  the  external  limiting 


Fig    259 -Diagram    sHowiNf.    the    Development   ov   the    Retinal 

Elements. 

external  limiting  memhranc-iKalhus,  ajtcr  Ca]al.) 

membrane,  beneath  which  they  form  a  defmite  layer  ir 
the  adult  In  the  mean  time  there  appears  opposite  the 
outer  end  of  each  cell  a  rounded  eminence  projecting  fron 
the  outer  surface  of  the  external  Vvmiuv^  membrane  intc 
the  pigment  layer.  The  eminences  over  the  cone  cells  an 
larger  than  those  over  the  rod  cells,  and  later,  as  both  in 


^4 


IIIK    OITIC    NKKVK. 


4S9 


crease  in  length,  they  become  recognizable  by  their  shape 
as  the  rods  and  cones. 

The  bipolar  cells  are  not  easily  distinguishable  m  the 
early  stages  of  their  differentiation  from  the  other  cells 
with  which  they  are  mingled,  but  it  is  believed  that  they 
are  represented  by  cells  which  are  bipolar  when  the  •   d 
and  cone  cells  are  still  in  a  unipolar  condition  (Fig.  25«>,  e). 
If  this  identification  be  correct,  then  it  is  noteworthy  that 
at  first  their  outer  processes  extend  as  far  as  the  external 
limiting  membrane  and  must  later  shorten  or  fail  to  elon- 
gate until  their  outer  ends  lie  in  what  is  termed  the  outer 
granular  layer  of  the  retina,  where  they  stand  in  relation 
to  the  inner  ends  of  the  rod  and  cone  cell  processes.     Of 
the  development  of  the  amacrinef/,  i)  and  horizontal  cells 
(n)  of  the  retina  little  h  known.     From  their  position  in 
new-born  kittens  it  seems  probable  that  the  former  are 
derived  from  cells  of  the  same  layer  as  the  ganglion  cells, 
while  the  horizontal  cells  may  belong  to  the  outer  layer. 

In  addition  to  the  various  nerve-elements  mentioned 
above  the  retina  also  contains  neuroglial  elements  known 
as  Miiiler's  fibers  (Fig.  259.  K),  which  traverse  the  entire 
thickness  of  the  retina.  The  development  of  these  cells 
has  not  vet  been  thoroughly  traced,  but  they  resemble 
closely  the  ependyr  'H  observable  in  early  stages  of 

the  spinal  cord.  ^        , 

The  Development  oj  the  Optic  Nerve.-Thi^  observations 
on  the  development  of  the  retina  have  shown  very  clearly 
that  the  great  majoritv  of  the  fibers  o:  '  iie  optic  nerve  are 
axis-cylinders  of  the  ganglion  cells  of  the  retina  and  grow 
from  these  cells  along  the  optic  stalk  toward  the  brain. 
Their  embrvonic  history  has  been  traced  most  thor- 
oughly in  rat  embryos  (Robinson),  and  what  follows  is 
based  upon  what  has  been  observed  in  that  animal. 

The  optic  stalk,  being  an  outgrowth  from  the  brain,  is  at 
41 


490 


TFIF.    DKVKI.OHMKNT    Ol-    THK    HUMAN    IIODV. 


I  i 

|4  - 


ill 


first  a  hollow  structure,  its  cavity  cotntnuiiicatin^  with 
that  of  the  third  ventricle  at  one  end  and  with  that  of  the 
optic  bulb  at  the  other.  When  the  chorioid  fissure  is  de- 
ve'  >ped,  it  extends,  as  has  already  been  d 'scriljed.  for 
some  distance  alonj;  the  posterior  surface  of  the  stall:  and 
has  lyini;  in  it  a  portion  of  the  hyaloid  artery.  Later, 
when  the  lips  of  the  fissure  fuse,  the  artery  becomes  en- 
closed within  the  stalk  to  form  the  urtnui  nnlralis  ntiua: 
of  the  adult  li'iK-  2()2).     By  the  formation  of  the  fissure 

the  orijjinal  cavity  of  the  ilistal 
portion    of    the    stalk    become^ 
obHterated,    and    at    the    same 
time  the  ventral  and  posterior 
walls  of   the   stalk   are  brought 
into  continuity  with  th     retinal 
layer  of  the  optic  cup,  and  so 
opportunity    is    };iven    for    the 
passage  of  the  axis-cylinders  of 
ihe    ganglion  cells   along   those 
walls   (Pig.   260).     At   an  early 
stage  a  section  of  the  proximal 
portion  of  the  optic  stalk  (Fig. 
261,  A)  shows  the  central  cavity 
surrounded  by  a  number  of  nu- 
clei   representing    the     mantle 
layer,  and  surrounding  these   a 
non-nucleated    laver    resembling    the    marginal    velum 
and  continuous  distally  with   the  similar  layer  of   the 
retina.     When  the  ganglion  cells  of  the  latter  begin  to 
send  out  their  axis-cylinder  processes,  these  pass  into  the 
retinal  marginal  velum  and  converge  in  this  layer  toward 
the  bottom  of  tlie  ciliary  fissure,  so  reaching  the  ventral 
wall  of  the  optic  stalk,  in  the  velum  of  which  they  may  be 
distinguished  in  rat  embryos  of  4  '""i--  and  still  more 


Fir,.     260.  -  Di.vr.RAMMATic 

Lo.NdlTUDINAL  SECTIO.V 
OF  THE  Ol'TIC  CUI'  AND 
STAUK     I'ASSIN<-.     THROtOH 

THE  Chorioid  Fissure. 
Ah,  Hyaloid  artery;  L,  lens; 
On,    fillers    of    the    optic 
ner^•»■;  Ox,  optic  stalk;  Ft, 
pigment  layer,  and  A',  re 
tinal  layer  of  the  retina. 


BFieVifiOflMW 


THK   OI'TIi:    NKKM 


491 


dcarlv  in  those  of  i)  nun.  (V^.  2M.  A).  Later,  as  the 
fibers'hecoine  more  tmmerous,  they  gradually  invade  the 
lateral  and  tinully  the  dorsal  walls  of  the  stalk,  and,  at  the 
same  time  the  mantle  eells  of  the  stalk  beeome  more  scat- 
tered ami  assume  the  form  of  eonnective- tissue  (neurog- 
lia) cells,  while  the  original  <avity  of  the  stalk  is  gradu- 
ally obliterated  ( I'ig.  2^>i ,  B).  Finally,  the  stalk  becomes 
a  solid  mass  of  nerve-fibers,  among  which  the  altered 
mantle  cells  are  scattered. 


Till-:  oi'TK  Stalk  of  Rat  Umbryos  ov  (.1)  •>  mm.  and  (n) 

-    (Rotniisoii.) 

Front  what  has  been  stated  ab(,ve  it  will  be  seen  that  the 
sens.,rv  cells  ..f  the  eye  belong  tc,  a  ^^"^^^^' ^'^'"'^H'S^^ 
from  those  of   the  other  sense-organs.     Ivmbrvologicall.    they 
a  e  a    penalized  portion  of  the  mantle  layer  of  tlu   mcc  u  lary 
canal,  whereas  in  the  other  organs  they  are  peripheral  structures 
cXt  representing  or  being  associated  with  represc-tnatne.  ol 
r^  stc^ior^root  ganglion  cells.     Viewed  from  th.s  slandjx.mt ,  and 
faking  into  considtraticm  the  fact  that  thcsensorv  portion  o     he 
retina  is  formed  from  the  invaginated  pari  ot  the  optic  bnlb 
some  light  is  thrown  upon  the  inverted   arrangement  of   the 
re  itn   ?iemcnts   the  rods  and  ones  being  directed  a^^ay  from 
The  source     f  light.     The  normal  relations  of  the  ntantle  layer 
and  rrginal  vdum  are  retained  in  the  retina,  and  the  latter 


492 


riiK  i)i:vi:i.oi'Mr.NT  oi'  the  human  iionv. 


1    e 


serving  as  a  conducting  layer  for  the  axis  cylinders  of  the  mantle 
layer  (ganglion)  cells,  the  layer  of  nerve  fibers  becomes  inter- 
posed between  the  source  of  light  and  the  sensory  cells.  Fur- 
thermore, it  may  be  pointed  out  that  if  the  differentiation  of  the 
retina  be  imagined  to  take  place  bef(jre  the  closure  of  the  medul- 
lary canal, — a  condition  which  is  indicated  in  some  of  the  lower 
vertebrates, — there  would  be  then  no  inversion  of  the  elements, 
this  peculiarity  being  due  to  the  conversion  of  the  medullar},- 
plate  into  a  tube,  and  UK^re  especially  to  the  fact  that  the  retina 
develops  from  the  outer  wall  of  the  optic  cup.  In  certain 
reptiles  in  which  an  eye  is  developed  in  connection  with  the  epi- 
physial «)Utgrowths  of  the  diencephalon,  the  letinal  porti(m  of 
this  pineal  eye  is  formed  from  the  inner  layer  of  the  bulb,  and 
in  this  case  there  is  no  inversion  of  the  elements. 

A  justificaticm  of  the  exclusion  of  the  optic  nerve  from  the 
category'  which  includes  the  other  cranial  nerves  has  now  been 
presented.  For  if  the  retina  be  regarded  as  a  portion  of  the 
central  nervous  system,  it  is  clear  that  the  nerve  is  not  a  nerve 
at  all  in  the  strict  sense  of  that  word,  but  is  a  tract,  confmed 
throughout  its  entire  extent  within  the  central  nervous  svstem 
and  comparable  to  such  groups  of  ril)ers  as  the  direct  cerebellar 
(jr  fillet  tracts  of  that  svstem. 


W 


Pi 

pi.  I 

ml 
•I  . 


The  Development  of  the  Vitreous  Humor. — It  has  already 
been  pointed  out  (p.  477)  that  a  blood-vessel,  the  hyaloid 
artery,  accompanied  by  some  mesodermal  tissue  makes 
its  way  into  the  cavity  of  the  optic  cup  through  the 
chorioid  fissure.  On  the  closure  of  the  fissure  the  artery 
becomes  enclosed  within  the  optic  stalk  and  appears  to 
penetrate  the  retina, upon  the  surface  of  which  itsbranches 
ramify.  In  the  embryo  the  artery  does  not,  however, 
terminate  in  these  branches  as  it  does  in  the  adult,  but  is 
continued  on  through  the  cavity  of  the  optic  cup  (Fig. 
262)  to  reach  the  lens,  around  which  it  sends  branches  to 
form  the  tunica  vasculosa  lentis. 

According  to  some  authors,  the  formation  of  the  vitre- 
ous humor  is  closely  associated  with  the  development  of 
this  artery,  the  humor  being  merely  a  transudate  from  it, 
while  others  liave  maintained  that  it  is  a  derivative  of  the 


Ui 


THE    VITRKOUS    HUMOX. 


493 


mesoderm  which  accompanies  the  vessel,  and  is  therefore 
to  be  regarded  as  a  pecuHar  gelatinous  form  of  connective 
tissue.  In  the  mammalian  eye  it  is  difficult  to  determine 
the  relative  merits  of  these  two  views,  but  the  fact  that 
in  the  lower  vertebrates— the  birds,  for  example— the 
vitreous  humor  forms  at  a  time  when  the  optic  cup  con- 
tains neither  mesoderm  cells  nor  blood-vessels  indicates  a 
probability  that  neither  of  them  is  ((uite  sufficient  to  ex- 
plain the  observed  phenomena.  Recently  it  has  been 
suggested  that  it  is  to  the  retinal  cells  that  one  must  look 


Fig.  262.-  Reco.nstruction  of  a  Portion  ok  thk  Kye  ok  .\n  Kmbryo 

OF    1.V8   MM. 

ah,  Hyaloid  artery;  cli,  cliorioid  coat;  /,  lens;  >,  retina.-  (///v.) 

for  the  formation  of  the  humor  (Rabl),  and  further  ob- 
servations along  this  line  are  desirable. 

Over  the  surface  of  the  vitreous  humor  a  structureless 
membrane,  known  as  the  hyaloid  membrane,  is  formed, 
apparently  by  a  condensation  of  the  vitreous  humor  or  as 
a  secretion  of  the  retinal  cells,  and  in  the  more  anterior 
portions  of  the  humor  fibers  appear,  extending  across 
from  the  ciliarv  processes  to  become  continuous  with  the 
capsule  of  the  lens  (Fig.  263,  si).  These  fibers  increase  in 
number  in  later  stages  and  represent  the  suspensory  liga- 
ment of  the  lens  [zonula  Zinnii),  and  spaces  which  occur 


494 


TllK    IIKVEIOI'MKNT    OF    THK    HUMAN    nOIlY, 


between  the  fibers  enlarge  to  produce  a  cavity  traversed 
by  scattered  fibers  and  known  as  the  canal  of  Petit. 

After  about  tne  third  month  the  portion  of  the  hyaloid 
artery  which  traverses  the  vitreous  humor  begins  to  un- 
dergo degeneration,  and  during  the  last  month  of  develop- 
ment it  disappears  altogether,  the  only  trace  of  its  exist- 
ence at  birth  being  a  more  fluid  consistency  of  the  axis  of 


;  i 


fi; 
si 


Fig.  26J.  -Transvickse  Section'  THROton  the  Ciliary  Re<;ion  of  a 
Chick  liMBRvo  of  Sixteen  U.ws. 

ac,  Anterior  cliaiiihtT  <if  the  eye;  </,  c  injunctiv.i ;  co,  cornea;  /,  iris;  /, 
lens;  mr,  ciliary  muscle;  rl,  retinal  layer  of  optic  cu]j;  sf,  spaces  of 
Fontana;  v/,  susi)ensory  ligament  of  the  lens;i',  vitreous  humor. — 
(Angtlucci.) 

the  vitreous  humor,  this  more  fluid  portion  representing 
the  space  originally  occupied  by  the  artery  and  forming 
what  is  termed  the  hyaloid  canal  (canal  of  Cloquei). 

The    DcTclopmvut  of  the  Cufcr  Coat  of  the    Eye,  of  the 
Cornea,   and  of  the  Anterior  Chamber. — Soon   after  the 


i,> 


Tin:    COKNKA. 


495 


formation  of  the  optic  bulb  a  condensation  of  the  meso- 
derm cells  around  it  occurs,  forming  a  capsule.  Over  the 
inner  portions  of  the  optic  cup  the  further  differentiation 
of  this  capsule  is  comparatively  simple,  resulting  in  the 
formation  of  two  layers,  an  inner  vascular  and  an  outer 
denser  and  fibrous,  the  former  becoming  the  chorioid 
coat  of  the  adult  eye  and  the  latter  the  sclerotic. 

More   externally,    however,    the    processes    are    more 
complicated.     After  the  lens  has  separated  from  the  sur- 
face ectoderm  a  thin  layer  of  mesoderm  grows  in  between 
the  two  structures  and  later  gives  place  to  a  layer  of  homo- 
geneous substance  ir  which  a  few  cells,  more  numerous 
laterally  than  at  the  center,    if    embedded.     Still  later 
cells  from  the  adjacent  mesenchyme  grow  into  the  layer, 
which  increases  considerably  in  thickness,  and   blood- 
vessels also  grow  into  that  portion  of  it  which  is  in  contact 
with  the  outer  surface  of  the  lens.     At  this  stage  the  in- 
terval between  the  surface  ectoderm  and  the  lens  is  occu- 
pied by  a  solid  mass  of  mesodermal  tissue  (Fig.  264,  co  and 
tv),  but  as  development  proceeds,  small  spaces  filled  with 
fluid  begin  to  appear  toward  the  inner  portion  of  the  mass 
iac),  and  these,  increasing  in  number  and  size,  eventually 
fuse'  together  to  form  a  single  cavity  which  divides  the 
mass  into  an  inner  and  an  outer  portion.     The  cavity  is 
the  anterior  chamber  of  the  eye,  and  it  has  served  to  sepa- 
rate the  coj-Hm  (co)  from  the  tunica  vasculosa  lentis  {tv), 
and,  extending  laterally  in  all  directions,  it  also  separates 
from  the  cornea  the  mesenchyme  which  rests  upon  the 
marginal  portion  of  the  optic  cup  and  constitutes  the 
stroma  of  the  iris.     Cells  arrange  themsei  "es  on  the  cor- 
neal surface  of  the  cavity  to  form  a  continuous  endothelial 
layer,  and  the  mesenchyme  which  forms  the  peripheral 
boundary  of  the  cavity  assumes  a  fibrous  character  and 
forms  the  liijamenlum  pectinatum  iridts,  among  the  fibers 


H 


mm 


it  'f 


496 


THK    nEVEI.OPMENT    OF    THE    HUMAN    BODY. 


of  which  cavities,  known  as  the  spaces  of  fontaua  (F'ig. 
263,  sf  ),  appear.  Beyond  the  margins  of  the  cavity  the 
corneal  tissue  is  directly  continuous  with  the  sclerotic, 
beneath  the  margin  of  which  is  a  distinctly  T  ickened 
portion  of  mesenchyme  resting  upon  the  ciliary  processes 
and  forming  the  stroma  of  the  ciliary  body,  as  well  as 


I 


ec- 


FiG.  264.    -Transverse  vSectiont  throu<;h  the  Ciuiary  Region  of  a 

^10  Embryo  of  23  mm. 
iic,  Arlcricjr  cliaiiiher  of  the  eye;  co,  cornea;  it,  ectoderm;  /.lens;  vie. 

ciliary  muscle;  />,  jjijiiiient  layer  of  tlie  optic  cup;  r,  retinal  layer; 

/:,  tunica  vasculos;i  lentis. — (Angelucci.) 


w 


giving  rise  to  the  muscle  tissue  which  constitutes  the 
ciliary  muscle  (Figs.  263  and  264.  mc). 

The  ectoderm  which  covers  the  outer  surface  of  the  eye 
docs  not  procet  1  beyond  the  stage  wlicn  it  consists  of 
several  layers  of  cdls,  and  never  develops  a  stratum  cor- 
neum.  In  the  corneal  region  it  rests  directly  upon  the 
corneal  tissue,  which  is  thickened  slightly  upon  its  outer 


THE    KVKI.IDS. 


497 


surface  to  form  the  mimlmnic  oj  Bowman:  more  ixripli- 
crally,  however,  a  (luantity  of  loose  mesodermal  tissue 
lies  between  it  and  the  outer  surface  of  the  sclerotic,  and, 
tojrcther  with  the  ectoderm,  forms  the  coujundiva  iVv^. 

263,  cj). 

The  Dcielopmcnt  oj  the  Aeeessory  Afjfyaratns  0}  the  Lye.- 
The  eyelids  make  tlieir  appearance  at  an  early  stai^e  as 
two  folds  of  skin,  one  a  sliort  distance  above  and  tlie  other 
below  the  cornea.  Tlie  center  of  the  folds  is  at  first  occu- 
pied bv  indilTerent  mesodermal  tissue,  wh.:h  later  be- 
comes modified  to  form  the  connective  tissue  of  the  lids 
and  the  tarsal  cartila,<;e,  tlie  muscle  tissue  probably  sec- 
ondarilv  .srrowin-  intr)  the  lids  as  a  result  of  tlie  spreading 
of  the  platvsma  over  the  face,  the  orbicularis  palpe- 
brarum  apparently  bein.s-   a  derivative  of  that  slieet  of 

muscle  tissue. 

At  about  tlic  beoinnini;  of  the  third  month  the  hds 
have  become  sutTiciently  lar<rc  to  meet  one  luiother,  where- 
upon  the  thickened  epithelium  which  iia>  formed  upon 
their  edi;es  unites  and  the  lids  fuse  to-ether,  in  which 
condition  thev  remain  until  shortly  before  birth.     Dunn.ij 
the  stase  of  fusion  the  eyelashes  i  I'ii;.  263.  // )  develop  at 
the  edi^es  of  the  lids,  havinir  ilic  same  developmental  liis- 
torv  as  ordinary  hairs,  and  from  the  fused  epithelium  of 
each  lid  ■ '  ere  grow  upward  or  downward,  as  the  case  ma> 
be,  into  the  mesodermic  tissue,  solid  rods  of  ectoderm, 
certain  of  wliicli  early  ,t,nve  olT  numerous  short  lateral 
processes   and    become    reco.iriii/.able   as    the    Mcilnnnian 
ijlamis  (m),  while  others  retain  the  simple  c..  limlrical  form 
and    represent    tlie    ,ilau,ls   oj    Moll.      When    the   eyelids 
separate,  these  solid  iii,i;rowths  become  '   >llow  by  a  break- 
ing down  of  their  central  cells,  just  as  in  the  sebaceous  and 
sudoriparous  -lands  of    the  >kin,  the  Meibomian  -lands 
beiiit,^  really  modifications  of  the  former  -lands,  wliile  the 
4- 


I 
1 


TffS**^ 


498 


THE    DEVELOPMENT   OF    THE    HUMAN    BODY. 


i 


(S  '. 


H 


glands  of  Moll  are  probably  to  be  regarded  as  specialized 
sudoriparous  glands. 

A  third  fold  of  skin,  in  addition  to  the  two  which  pro- 
duce the  eyelids,  is  also  developed  in  connection  with  the 
eye,  forming  the  plica  semiluuaris.     This  is  a  rudimentary 


ma 


Fig.  265. —Section  through  the  Margins  of  the  Fiseu  Eyelids  in 

AN  Embryo  ok  Six  Months. 
h,  eyelash;   //,  lower  lid;  w,  Meibomian  gland;   mu,  muscle  bundle;   id, 

upper  lid. — (Sch-ueigger-Scidl.) 


third  eyelid,  representing  the  nictitating  membrane  which 
is  fairly  well  developed  in  many  of  the  lower  tnammals 
and  especially  well  in  birds.  In  man  a  number  of  glands 
develop  in  its  substance,  forming  a  small  reddish  nodule 
known  as  the  caruncula  lachrymalis. 

The  lachrymal  gland  is  developed  at  about  the  third 


UaM 


THE  LACHRYMAL  APPARATUS. 


499 


month  as  a  number  of  branching  outgrowths  of  the  ecto- 
derm into  the  adjacent  mesoderm  along  the  outer  part  of 
the  line  where  the  epithelium  of  the  conjunctiva  becomes 
continuous  with  that  covering  the  inner  surface  of  the 
upper  eyeUd.  As  in  the  other  epidermal  glands,  the  out- 
growths and  their  branches  are  at  first  solid,  later  becom- 
ing hollow  by  the  degeneration  of  their  axial  cells. 

The  lachrymal  or  nasal  duct  is  developed  in  connection 
with  the  groove  which,  at  an  early  stage  in  the  develop- 
ment (Fig.  52),  extends  from  the  inner  corner  of  the  eye  to 
the  olfactory  pit  and  is  bounded  posteriorly  by  the  maxil- 
lary process  of  the  first  visceral  aich.     The  epithelium 
lying  in  the  floor  of  this  groove  thickens  toward  the  begin- 
ning of  the  sixth  week  to  form  a  solid  cord,  which  sinks 
into  the  subjacent  mesoderm,  though  retaining  connection 
with  the  ectoderm  at  either  end;  its  upper  end  is  con- 
tinuous with  the  ectoderm  of  the  edge  of  the  upper  eyelid, 
while  the  lower  one  is  united  with  that  of  the  olfactory  pit. 
Later,  the  solid  cord  acquires  a  lumen,  and  from  its  pal 
pebral  end  a  bud  arises  which  unites  with  the  ectoderm 
of  the  edge  of  the  lower  eyelid  and  produces  the  lower  limb 
of  the  lachrvmal  canal. 


H 


LITERATURE. 

A.  ANGE1.UCC1:  "Ueher  Knlwickelung  and  Bau  dcs  vorderen  Lvealtractus 
der  Vertebraten."  x\rchiv  jiir  mikrosk.  A nat.,  xix,  188 1 . 

J.  BAGINSKY-.  "Zur  Kntwickelung  der  Gclu.rschneckc,"  Archiv  jur  mik- 
rosk. .4 mi/.,  XXVIII,  1886. 

I.   Broman:  "Die  Entwickelungsgeschichte  der  Gehnrkm.chelchen  beim 

Menschen,"  Anal,  llcftc,  xi,  1898.  ,  .      ,     .  , 

S   R\MON  yCajau:    "  Xouvelles  cmtrilnitions  a  I'etude  lustologique  de 

la  rcline,"  /ourn.  de  fAmit.  ci  dc  la  PhyswL,  xxxii,  1806. 
1    DissE-  ■•  Die  erste  Kntwickelung  der  Rieclmerven,"  Amit.  ll,jte,  ix,  18«>/ . 
I.  ciKABKKU :    •  Heiuugc  zut  (k•tu•^c  des  Ck>scl„nacks..rgans  der  Menschen. 

Morhhol.  Arlhiten,  viii,  1898. 
J.   A    Hammar:  "Zur  allgenieinen   Morpliologie  der  Schlundspalten  des 


r?" 


&-t^  *is^:^ 


500 


THE    DEVEI.OPMKNT    OK    THE    HUMAN    UODY. 


m  \ 


Mcnschen.     Zur  EntwickcltingSRescliichte  des   Mittelohiraumes,  des 

iiusscren  (lehorganges  und  des  Faukenfellcs  btim  Mensclien,"  Anat. 

Anztiger,  xx,  1901 
HeekfordT:  "Studien  iihcr  den  Muse,  dilatator  puptlla-  saniiiit  Angahe 

von  geineinsehafllicher  Kenn/eielien  einiger  Falle  ei)itlielialer  Miis- 

culalur,"  Aunt.  Ilrjlc,  XIV. 
f.  HKiiETSCHWEiUKR:  "  Die  enil)ryol(.>,'iselie  IvntwickelunKdesSteiKliUKcls." 

Archil  jitr  Atutl.  uml  ."hyuol.,  Atint.  Ahtli.,  18'>«. 
\V.  His,   |R.  :  "Die  KntwickehingsKeseliiehte  des  Aeiistico-FaeialisKehietes 

t)eiiii  Menselien,"  Arcliir  fur  Aiuil.  uml  Physiol.,  Anal.  AlUh.,  Supple- 
ment, 18<>7. 
y.    VON    MiH.\LK()vicz;  "  Nasenliolile    und   Jacohsonsches   Organ.      ICine 

morpliologische  Sludie,"  Anal.  Ihjtc,  xi,  1898. 
C.  Rabl:  "l'el)er  den  Hau  und  Kntwickelung  der  Linse,"   '/.titschrtjt  jiir 

u-is.umch.  Zoolof,ic,  LXii  and  Lxv,  1898;  LXVii,  1899. 
A  Robinson:  "On  the  Formation  and  Structure  of  the  Optic  Nerve  and  Its 

Relation  to  the  ( )ptic  Stalk,"  Journal  oj  Anal,  and  Phyxiol.,  XXX,  1896. 
SiEbEnmann:  "Die  ersten  Anhigen  voni  Mittelohrrauni  und  Oehorknoch- 

elchen  des  nienschlichen  Hnihryo  in  der  4  bis  6  Woche,"  Archiv  jitr 

Anat.  und  l'hy.uol.,  Anal.  Ahlh.,  1894. 
A.  SziLi:  "Zur    Anatoniie    und    Kntwickelungsgeschichte   der   hinteren, 

Irisschichten,  niit  hcsonderer  Hr-rucksiclitiRung  des  Musculus  sphincter 

iridis  des  Menschen,"  .\nal.  .Anzrinvr,  XX,  1901. 
V.  TfCKERMAN:    "On  the   Dcvelo])tTient   of    the  Taste  Organs  in  Man," 

Journal  of  .\nal.  and  Physiol.,  xxiv,  1889 


CHAITKR  XVI. 
POST-NATAL    DEVELOPMENT. 

In  the  preceding  pages  attention  has  been  cUreeted 
prineipally  to  the  changes  which  take  place  in  the  various 
organs  during  the  period  before  birth,  for,  with  a  few  ex- 
ceptions, notably  that  of  the  liver,  the  general  form  and 
histological  peculiarities  of  the  various  organs  are  ac- 
quired before  that  epoch.  Development  does  not.  how- 
ever cease  with  birth,  and  a  few  statements  regardmg 
the  changes  which  take  place  in  the  interval  betvveen 
birth  and  maturity  will  not  be  out  of  place  m  a  work  ol 

this  kind.  .      .^ 

The  conditions  which  obtain  during  embryonic  lite  are 
so  different  from  those  to  which   the  body  must  later 
adapt  itself,  that  arrangements,  such  as  those  connected 
with  the  placental  circulation,  which  are  of   fundamen- 
tal   importance    during    the    life    iu    utero,  become   of 
little  or  no  use,  while  the  relative  importance  of  others 
is  greativ   diminished,  and    these   changes   react    more 
or  less  profoundlv  on  all  parts  of  the  body.     Heiice,  al- 
though the  post-natal  development  consists  chiefly  in  the 
growth  of  the  structures  formed  during  earlier  stages,  yet 
the  -rowth  is  not  equallv  rapid  in  all  parts,  and  indeed  in 
some  organs  there  mav  even  be  a  relative  decrease  in  sue. 
That  this  is  true  can  be  seen  from  the  annexed  figure  (Mg. 
266^   which  represents  the  body  of  a  child  and  that  of  an 
aduU  man  drawn  to  the  same  scale.     The  greater  relative 
size  of  the  head  and  upper  part  of  the  body  in  the  child  is 
very  marked,  and  the  central  point  of  the  height  of  the 

501 


!l 


1 


502 


THE    DEVELOPMENT    OF     THE    HUMAN    HOPY. 


child  is  situated  at  about  the  level  of  the  umbilicus,  while 
in  the  man  it  is  at  the  symphysis  pubis.  This  excessive 
development  of  the  upper  portions  of  the  body  of  the  child 
is  largely  due  to  the  nature  of  the  blood-supply  during 
fetal  life,  when,  as  may  be  seen  by  reference  to  Fig.  152, 
the  blood  passing  to  the  head,  neck,  arms  and  upper  por- 


Fir,.  266. — Child  and  Man  Drawn  to  the  Same  Sc.\uK.^(.Langer,  from 
the  "Growth  oj  the  Brain,"  Contemporary  Science  Series,  by  permission 
oj  Charles  Scribner's  Sons.) 

tions  of  the  thorax  leaves  the  aorta  before  the  ductus 
arteriosus  opens  into  it,  and  is  therefore  practically  un- 
mixed with  venous  blood,  while  throughout  the  rest  of  the 
V)ody  the  supply  is  largely  diluted  with  blood  from  the 
right  side  of  the  heart. 

That  there  is  a  distinct  change  in  the  geometric  form  of 


I'OST-NATAI.    DF.VKI.OI'MKNT. 


503 


the  body  during  growth  is  also  well  shown  by  the  folloNN- 
jne  consideration  (Thoma).     Taking  the  average  height 
of  a  new-born  male  as  s<^  nun.,  and  that  of  a  man  of 
thirty  years  of  age  as  1686  mm.,  the  height  »    ^'h- J)odv 
will  have  increased  from  birth  to  adolescence  ,'.,.,'  -  ^■^^ 
times.     The  child  will  weigh  31  kilos  and  the  man  f,6.. 
kilos,  and  if  the  speciHc  gravity  of  the  body  with  the  in- 
cluded gases  be  taken  in  the  one  case  as  0.90  and  in  the 
other  as  0.9^.  then  the  volume  of  the  child's  body  will  be 
.,  44  Uters  and  thai  of  the  mans  7....S  liters,  and  the  in 
crease  in  v.)lume  will  be  -;•;        20.66.     If.  new.  the  m- 
crease  in  volume  had  Uiken  place  without  any  alteration 
in  the  geometric  form  of  tlie  bodv.  it  should  be  equal  to 
the  cube  of  the  increase  in  height;  this,  however,  is  .V37 
.-  38.27.  a  num!)er  well-nigh  twice  as  large  as  the  actual 

'"  BuTTn  addition  to  these  changes,  which  are  largely 
dependent  upon  ditlerences  in  the  supply  of  nutrition 
there  are  others  associated  with  alterations  in  the  general 
metabolism  of  the  body      Up  to  adult  Ufc  the  cons  ruc- 
tive  metabolism  or  anabolism  is  in  excess  of  the  des  ruc- 
tive  metabolism  or  kataboUsm,  but  the  amount  of  the 
excess  is  much  greater  during  the  earlier  periods  of  devel- 
opment and  gradually  diminishes  as  the  adult  condition 
is  approached.     That  this  is  true  during  intrauterine  hfe 
is  shown  bv  the  following  figures,  compiled  by  Donaloson : 


Wku.ht  in  Crams.        Agk  in  Wkkks.    Wkioht  iN  Grams. 

o.oonC) 


AliK  IN  Wkeks. 

Wku.ht 

0  (DViini) 

0 

4 

8 
12 

4 

20 

120 

20 

285 

24 

6.VS 

28 

1220 

,^2 

1700 

ih 

2240 

40  (l)inh) 

.^2.S0 

ii   i 


5^1 


TIIK    DI.VKHtlMI.Nl     Ol      rill.    HUMAN    l!«H)V. 


I'roin  this  tahli-  it  may  Ik-  sirii  that  tin-  iinhryo  o 
(.•i.i,'lit  wtvks  is  six  tlKiiisiiul  <ix  Iiundrcd  and  sixty  stvfi 
tiiiit's  as  lu'avN  as  tlu'  ovum  from  wliicli  it  started,  atK 
if  tlic  increase  of  n.owtli  for  each  of  the  succcicHnj,'  juriod 
of  four  wcfks  Ik'  riprtsintt'd  as  jU'rcTiitams,  it  will  be  seei 
that  the  rate  of  increase  uiiderijois  a  ra|)i<l  dimimitioi 
after  the  sixteenth  week,  and  from  that  on  diininishe 
,i;radually  but  less  rapidly,  the  figures  beini;  as  follows: 


I'KKIc 

|.S    I.| 

I'm. 

INIAl.l- 

I'KHKMis   i.F. 

I'KK 

CN  I  rV<.l 

\\f 

Iks. 

Is. 

Kl  ASI' 

WlUks, 

In. 

KKAS!-:, 

H 

12 

400 

J  4  :s 

'»? 

\2 

W> 

SOO 

JS     ij 

Vt 

\u 

JO 

\M 

?J    u, 

M 

JO 

2i 

12? 

M,   40 

45 

in 

ii  I 
IP  I 

Ii      i 


It  I     I 


That  the  same  is  true  in  a  t^eneral  way  of  the  growtl 
after  birtii  maybe  seen  from  the  table  on  paije  505.  repre 
sentini;  tJie  a\en),i,a*  wei,i,dit  of  the  body  in  h'ns^lish  male 
at  dilTerenl  years  from  birth  up  tt)  twenty-three  1  Roberts) 
and  also  the  percenta,i,a'  rate  of  increase. 

Certain  interestinj;  peculiarities  in  ])ost-n;.tal  j^rowtl 
become  apparent  from  an  examinati(jn  of  this  table.  I'o 
while  there  is  a  general  diminution  in  the  rate  of  growth 
yet  there  are  marked  irregularities,  the  most  noticeabl 
being  (n  a  rather  marked  diminution  during  the  eleventl 
and  twelfth  >ears,  followed  by  (2)  a  rapid  aceeleratioi 
which  reaches  its  maximum  at  about  the  sixteenth  yea 
and  then  very  rapidly  diminishes.  These  irregularitie 
may  be  more  clearly  seen  from  the  following  charts,  whicl 
represent  tlie  curves  oblainetl  by  plotting  the  annua 
increase  of  weiglit  in  boys  (Chart  I;  and  girls  (Chart  IIj 
The  diminution  and  acceleration  of  growth  referred  t( 


^'^^^    '-^ 


ihryo  of 
ty  SI' VI' II 
tc'd,  and 
:;  jHTiods 

1  1)1'  Sl'CU 

nitnitioti 
miiiislu's 
•Hows: 


POSI-NMAI.    PK.VI  lOl'MKNT. 


505 


KK 

KNr^t.i 

M 

KKAS!', 

'>J 

i') 

\2 

4.S 

'  cjrowtli 
i.S,  rcprc- 
■^h  mak'S 
loherls), 

t^rowtli 
le.  For 
growth, 
)tic(.'abk' 
(.'k'vetitli 
.'k-ralion 
tith  year 
;ularilies 
s,  which 
•  annual 
liart  IIj. 
crrt'd  to 


above  are  clearly  observable,  and  it  is  interesting  to  note 
that  they  tK-cur  at  earlier  j)erio(ls  in  girls  than  in  boys,  the 
diminution  occurring  in  girls  at  the  eighth  and  ninth 
years  and  the  acceleration  reaching  its  tnaxinmni  at  the 
thirteenth  vear. 


VhaH. 


0 
1 

2 
,; 
4 
.s 
(• 

,s 
<» 

1(1 
II 
1: 
1.^ 

14 
15 
1(. 
17 
IS 
l'» 
20 
21 


Ni  Mill  K  111'  Casus. 


4S1 

2 
4\ 

1(12 

l'>.> 

224 

24*. 

H2i) 

142.=' 

14(.4 

1  5'^") 

i:h(. 

244^ 

l')r>2 
■MIS 
22.VS 
24')*. 

2i.s(; 

14.<8 
S.M 
7.<S 
=i42 
,S.S1 


VVkii.m 

IN 

Kll.l.l.RAMS 

^ 

2 

(10 

H) 

14 

7* 

l.-^ 

4 

Hi. 

>> 

18. 

I 

20 

J 

22 

6 

24 

1) 

27 

4 

,V) 

*. 

M 

(1 

,U 

') 

37 

*> 

41 

t 

46 

*. 

5.^ 

.<» 

59 

_  .> 

f.2 

2 

*i.^ 

4 

(.4 

') 

fi.S 

7 

(.7 

0 

(<7 

(1 

l'IRl-|:Nl*l.K 

1m.  RKASK. 

(238) 

(,U.) 

* 

4 

H* 

<> 

1 

11 

1 

12 

4 

10 

1 

10 

11 

,s 

(> 

1  . 

5 

t 
10 

4 

11 

~ 

l.s 

1 

10 

4 

<> 

1 

.9 

2 

5 

1 

.2 

1 

.9 

0 

Considering,  now.  merely  tlie  general  diminrtion  in  the 
rate  of  growth  wliicli  occurs  from  birth  to  tici.lt  life,  it 

♦  Krotn  a  comparison  with  oilier  similar  tnhk-s  tlicrt  i^  litlU  douht  hut 
tliat  the  weight  given  above  for  the  second  year  is  too  hi-h  to  be  accepted 
as  a  Rood  average.  Conse<|uently  the  percentage  increase  for  '  le  second 
year  in  tuo  high  and  that  f^.r  tlu-  third  year  Iol.  l-.v-- 

It  may  be  mentioned  that  the  weights  in  the  original  lalile  are  expressed 
in  pounds  avoirdupois  and  have  been  here  converted  into  kilograms,  and 
further  the  figures  representing  the  percentage  increase  have  been  added. 


5o6 


THE    DEVELOPMENT    OF   THE    HUMAN    liODV. 


becomes  interesting  to  note  to  what  extent  the  organs 
which  are  more  immediately  associated  with  the  meta- 
bolic activities  of  the  body  undergo  a  relative  reduction 
in  weight.     The  most  important  of  these  organs  is  un- 


II 


I'     I 


V-- 


li'<        i 
l!:# 


a       9      JO      //_    K      13      l*t%t6      17J8 


Fig  267  —Curves  Showing  the  Annual  Increase  in  Weight  in  (I) 

Boys  and  (II)  Girls. 

The  faint  line  represents  the  curve  from  British  statistics,  the  dotted 
line  that  from  American  (Bowditch),  and  the  heavy  hne  the  aver- 
age of  the  two.  Before  the  sixth  year  the  data  are  unreliable.  - 
(Slcphenson.) 

doubtedly  the  liver,  but  with  it  there  must  also  be  con- 
sidered the  thyreoid  and  thymus  glands,  and  probably  the 
suprarenal  bodies.     In  all  these  organs  there  is  a  marked 


POST-NATAL    DKVKLOPMENT. 


507 


diminution  in  size  as  compared  with  the  weight  of  the 
body,  as  will  be  seen  from  the  following  table  (H.  \  icr- 
ordt),  which  also  includes  data  regarding  other  organs  in 
which  a  marked  relative  diminution,  not  in  all  cases 
readily  explainable,  occurs : 

ABSOLUTE  WEIGHT  IN  GRAMS 
New-born  and  Auult. 


L,VBR.     :2!!r:Zrr     ^^I^^I^'^^-S-uekn.  Hkak..     ^_       Bka.n. 


RKOID.        MU3.       AI.  BODIKS. 

05 


141.7        4.85      8.15 
1819.0      33.8      26.9 


10.6 


Spinal 
Cord. 


23  6      23.3      381.0       5.5 
7  A        163.0  1300.6,305.9     14.W.9     39.15 

I  ! 


PERCENTAGE  WEIGHT  OF  ENTIRE  BODY 
New-born  and  Adui<t. 


Liver         "T"'-       '^"^'    ^"''«*"t;r  Spleen.  Heart.     ^^^^ 
L,ivEK.        REoiD.       MLS.     al  Bodies.  nuvs. 


Spinal 


Brain.   ,  ^ord 


4.57         0.16 
2.75         0.05 


0.26 
0.04 


0  23    '  0.34      0.76     0.75      12.29        0.18 
0.01       0.25      0.46      0.46  j     2.16  ]     0.06 


On  the  other  hand,  the  remaining  organs,  when  com- 
pared with  the  weight  of  the  body,  cither  show  an  increase 
or  remain  practically  the  same. 

ABSOLUTE  WEIGHT  IN  GRAMS. 
New-born  and  Adult. 


Skin  andSi-bci- 
TANEois  Tis- 
sues. 


Skelkton 


611.75      425.5 
11765.0    i  11575.0 


MUSCILA- 
TURE. 


776.5 

28732.0 


iS  T  0  M    AC  H 

:     AND  INTKS-     Pancreas. 
!      TINKS. 


65 
1364 


3   5 
97.6 


Lungs. 


54.1 

994.9 


h 


■^w^^WBj 


.ikSJ 


I 


1      ' 


If 


I 


508  THE    OEVKLOPMENT    OF    THE    HUMAN    BOUV. 

IM'RCKNTAGE   oF   liODV-WI'IOIlT. 
New-born  ani>  Anri.T. 


Skin  and  Sibcu- 


.,  S  r  ')  M  A  c  H 

TANKOL-S       TIS-      SKKIKTON.  ,,,^^.  ANDlMI-S        ^A^^^KA^,. 

SUES. 


1  INKS. 


19.73 
17.77 


1,^.7 
17.48 


2.^  OS 
4.^ .  40 


2.1 

2  06 


0.11 
0.1. s 


Lungs. 


1.7.S 
1  .  50 


From  this  table  it  will  be  seen  that  the  greatest  incre- 
ment of  weight  is  that  furnished  by  the  muscles,  the  per- 
centage weight  of  which  is  one  and  three-fourths  times  as 
great  in  the  adult  as  in  the  child.     The  difference  does  not, 
iiowever,  depend  upon  the  dilTerentiation  of  additional 
muscles;  there  are  just  as  many  muscles  in  the  new-born 
child  as  in  the  adult,  and  the  increase  is  due  merely  to  an 
enlargement  of  organs  already  present.     The  percentage 
weight  of  the  digestive  tract,  pancreas,  and  lungs  remains 
practically  the  same,  while  in  the  case  of  the  skeleton 
there  is  an  appreciable  increase,  and  in  that  of  the  skin 
and  subcutaneous  tissue  a  slight  diminution.  The  latter  is 
readily  understood  when  it  is  remembered  that  the  area  of 
the  skin,  granting  that  the  geometric  form  of  the  body 
remains  the  same,  would  increase  as  the  square  of  the 
length,  while  the  mass  of  the  body  would  increase  as  the 
cube,  and  hence  in  comparing  weights  the  skin  might  be 
expected  to  show  a  diminution  even  greater  than  that 
shown  in  the  table. 

The  increase  in  the  weight  of  the  skeleton  is  due  to  a 
certain  extent  to  growth,  but  chiefly  to  a  completion  of 
the  ossification  of  the  cartilage  largely  present  at  birth. 
A  comparison  of  the  weights  of  this  system  of  organs  does 
not,  therefore,  give  evidence  of  the  many  changes  of  form 
which  may  be  perceived  in  it  during  the  period  under 


UL^j^ 


POST-NATAL    PEVEI.OPMKNT. 


509 


consideration,  and  attention  may  be  drawn  to  some  of  the 
more  important  of  these  changes. 

In  the  spinal  column  one  of  the  most  noticeable  pecu- 
liarities observable  in  the  new-born  child  is  the  absence 
of  the  curves  so  characteristic  of  the  adult.    These  curves 


Fig.  26. 


LO.NGITUDINAU    S-'CTION     THROUGH     THE    S.\CRi;M    OH    A    NEW- 

BOK.N  Female  Child.— (/•"f///iwg.) 


are  due  partlv  to  the  weight  of  the  body,  transmitted 
through  the  spinal  column  to  the  hip-joint  in  the  erect 
position,  and  partly  to  the  action  of  the  muscles,  and  it  is 
not  until  the  erect  position  is  habitually  assumed  and  the 
musculature  gains  in  development  that  the  curvatures 
become  pronounced.     Kven  the  curve  of  the  sacrum,  so 


5IO 


THE    DEVELOPMENT    OK    THE    HUMAN    BODY. 


■i 


marked  in  the  adult,  is  but  slight  in  the  new-born  child, 
as  may  be  seen  from  Fig.  268,  in  which  the  ventral  surfaces 
of  the  first  and  second  sacral  vertebra?  look  more  ventrally 
tlian  posteriorly,  so  that  there  is  no  distinct  promontory. 
But,  in  addition  to  the  appearance  of  the  curvatures, 
other  changes  also  occur  after  birth,  the  entire  column 
becoming  much  more  slender  and  the  proportions  of  the 
lumbar  and  sacral  vertebrae  becoming  quite  different,  as 
n^ay  be  seen  from  the  following  table  f  Aeby) : 

I  KXGTHS    OF    THE    VERTEBRAL  REGIONS   EXPRESSED  AS 
PERCENTAGES  OF  THE  ENTIRE  COLUMN. 


loRACIC. 

Lumbar 

47.:. 

26.8 

46.7 

30.0 

45.6 

.^4.2 

47.2 

M .  1 

46.6 

31.6 

AOd.  LKRVICA 

New-horn  child, 25  .6 

Male    2  years,    2.^.3 

••       5  '   "                                      ....  20.3 

"     11       •■        : 19.7 

•'     adult 22.1 


The  cervical  region  diminishes  in  length,  while  the  lum- 
bar gains,  the  thoracic  remaining  approximately  the  same. 
It  may  be  noticed,  furthermore,  that  the  difference  be- 
tween the  two  variable  regions  is  greater  during  youth 
than  in  the  adult,  a  condition  possibly  associated  with 
the  general  more  rapid  developmen  .  of  the  lower  portion 
of  the  body  made  necessary  by  its  imperfect  development 
during  fetal  life.  The  difference  is  due  to  changes  in  the 
vertebra?,  the  intervertebral  disks  retaining  approxi- 
mately the  same  relative  thickness  throughout  the  period 
under  consideration. 

The  form  of  the  thorax  also  alters,  for  whereas  in  the 
adult  it  is  barrel-shaped,  narrower  at  both  top  and  bottom 
than  in  the  middle,  in  the  new-born  child  it  is  rathei  coni- 
cal, the  base  of  the  cone  being  below.     The  difference 


POST-NATAI.    UEVELOPM ENT. 


511 


depends  upon  slight  differences  in  the  form  and  articula- 
tions of  the  ribs,  these  beins  more  hor'  ontal  in  the  child 
and  the  opening  of  the  thorax  directed  more  directly  up- 
ward than  in  the  adult. 

As  regards  the  skull,  the  processes  of  growth  are  very 
complicated.     Cranium  and  brain  react  on  one  another, 
and  hence,  in  harmony  with  the  relatively  enormous  size 
of  the  brain  at  birth,  the  cranial  cavity  has  a  relatively 
greater  volume  in  the  child  than  in  the  adult.     Tlic  fact 
that  the  entire  roof  and  a  c(.nsiderablc  part  of  the  sides  of 
the  skull  are  formec'  of  membrane  bones  which,  at  birth, 
are  not  in  sutural  contact  with  one  another  throughout, 
gives  opportunitv  for  considerable   modifications,  and, 
furthermore,  the  base  of  the  skull  at  the  early  stage  still 
contains  a  considerable  amount  of  imossitkd  cartilage. 
Without  entering  into  minute  details,  it  may  be  stated 
that  the  principal  general  changes  which  the  skull  under- 
goes in   its  post-natal   development   are    d)  a   relative 
elongation  of  its  anterior  portion  and  (2)  an  increase  in 
the  relative  height  of  the  superior  maxilla\ 

If  a  line  be  drawn  between  the  central  points  of  the  oc- 
cipital condyles,  it  will  divide  the  base  of  the  skull  into  two 
portions,  which  in  the  child's  skull  are  equal  in  length. 
The  portion  cf  the  skull  in  front  of  a  similar  line  in  the 
adult  skull  is  very  much  greater  than  that  which  lies  be- 
hind, the   proportion  between  the  two  parts  being  5 :  3, 
a^r  linst  3 :  3  in  the  child  (Froriep).     There  has,  therefore, 
been  a  decidedly  more  rapid  growth  of  the  anterior  portion 
of  the  skull,  a  growth  which  is  associated  with  a  corre- 
sponding increase  in  the  dorso-ventral  dimensions  of  the 
superior   maxilla\     These   bones,   indeed,   play   a   very 
important  part  in  determining  the  proportions  of  the 
skull  at  dilTereiit  periods.     They  arc  so  intimately  asso- 
ciated with  the  cranial  portions  of  the  skull  that  their 


,_JBGaKI" 


mw9>>NipKW0i*^:wr-'  •  •w 


512  THE    I)i:VEI.OPMENT    OF    TllK    HUMAN    liODY. 

increase  necessitates  a  corresponding  increase  in  the  ante- 
rior part  of  the  cranium,  and  their  increase  =n  this  direc- 
tion stands  in  relation  to  the  development  of  the  teeth, 
the  eight  teeth  which  arc  developed  in  each  maxilla  (in- 
cluding the  premaxilla  in  the  adult  requiring  a  longer 
bone  tlian  do  the  five  teeth  of  the  primary  dentition,  these 
again  requiring  a  greater  length  when  completely  devel- 
oped than  they  do  in  their  immature  condition  in  the  new- 
born child.  .  , 
But  far  more  striking  than  the  dilTerence  just  described 
is  that  in  the  relative  height  of  the  cranial  and  facial 


I 


>n  » 


I 


p„.  269  -Skull  »k  a  New-born  Child  and  of  an  Adult  Man,  Drawn 
Ho.  26>.     ^^^''j^  J^„^j,,^^ELY  THE  Same  ScAUE.-{Henke.) 


■)     I 


'A  i 


3 


t1*  ! 


regions  (Fig.  269).  It  has  been  estimated  that  the  vol- 
umes of  the  two  portions  have  a  ratio  of  8:  1  in  the  new- 
born child,  4:  I  at  hve  years  of  age,  and  2 :  1  in  the  adult 
skull  (Froriep),  and  these  differences  are  due  principally 
to  changes  in  the  vertical  dimensions  of  the  superioi 
niaxiUa-^  As  with  the  increase  in  length,  the  increase  uonn 
under  consideration  is,  to  a  certain  extent  at  least,  asso 
ciated  with  the  development  of  the  teeth,  these  structure' 
calling  into  existence  the  alveolar  processes  which  an 
practically  wanting  in  the  child  at  birth.     But  a  mor. 


DL^ 


FOST-NATAI.    nr.VF.I.Ol'Mr.N'l' 


5>3 


impor^ant  factor  is  the  development  in  the  maxiUce  of  the 
antni  of  Highmore.  the  practically  solid  bodies  o    the 
bones  becoming  transformed  into  hollow  shells.      Ihese 
cavities,  together  with  the  sinuses  of  the  sphenoid  and 
frontal  bones,  which  are  also  post-natal  developments, 
seem  to  stand  in  relation  to  the  increase  in  length  of  the 
anterior  portion  of  the  skull,  serving  to  diminish  the 
weight  of  the  portion  of  the  skull  in  front  of  the  occipital 
eondvles  and  so  relieving  the  muscles  of  the  neck  of  a  con- 
siderable strain  to  which  they  would  otherwise  be  sub- 
jected, f 
These  changes  in  the  proportions  of  the     cull  have,  oi 
course,  much  to  do  with  the  changes  in  the  general  pro- 
portions of  the  face.     Hut  the  changes  which   take  place 
in  the  mandible  are  also  important  in  this  connection 
and  are  similar  to  those  of  the  maxilla-  in  being  associated 
with  the  development  of  the  teeth.     In  the  new-born 
child  the  horizontal  ramus  is  proportionately  shorter  than 
in  the  adult,  while  the  vertical  ramus  is  very  short  and 
joins  the  horizontal  one  at  an  obtuse  angle.     The  develop- 
ment of  the  teeth  of  the  primary  dentition,  and  later  of 
the  three  molars,  necessitates  an  elongation  of  the  hori- 
zontal ramus  equivalent  to  that  occurring  in  the  maxiUs, 
and  at  the  same  time,  the  separation  of  the  alveolar  bor- 
ders of  the  two  bones  requires  an  elongation  of  the  vertica. 
ramus  if  the  condyle  is  to  preserve  its  contact  with  the 
glenoid  fossa,  and  this,  again,  demands  a  diminution  of  the 
angle  at  which  the  rami  join  if  the  teeth  of  the  two  jaws 
are  to  be  in  proper  apposition. 

In  the  bones  of  the  appendicular  skeleton  secondary 
epiphysial  centers  play  an  important  part  in  the  ossifica- 
tion and  in  few  are  these  centers  developed  prior  to 
birth  while  the  union  of  the  epiphyses  to  the  main  por- 
tions' of  the  bones  takes  place  only  toward  maturity. 
43 


■^^•J'.^r: . ;  ^^wFji;-'  c^sr ' 


5>4 


THE    DEVRI.OPMKNT    Ol"   THE    HUMAN    liOnV. 


m 


The  dates  at  which  the  various  primary  and  secondary 
centers  appear,  and  the  time  at  which  they  unite,  may 
be  seen  from  the  following  table : 


UPPER  EXTREMITY. 


Bone. 


Apfkaranch    of 
Primary  Ckn- 

I  TKR.  I 


Clavick',    .  . 
Scapula,    .  . 

Hody,  . . . 

Coracnid, 


Hunicruv 


Ulna,    .  . 
Radius, 


Os  magnum, 
Unciform,   . 
Cuneiform, 
Semilunar, 
Trapezium, 
Scaphoid,    . 
Trapezoid,  . 
Pisiform,  .  . 
Metacarpals, 
Phalanges, 


Otii  urek. 

8tlt  week 
1st  year. 

Hlh   urrk.       - 

&th  week. 
8th  week. 


1st  year. 
2d  year. 
.3d  year. 
4th  year. 
5  th  year. 
6th  year. 
8th  year. 
12lli  vear. 
8th  week. 
Bth-lOth  week. 


.\l'l'EARANCE  OF  SECONDARY 

Centers. 


Fusion  of  Cen- 
ters. 


(At  sternal  end)  17tii  year.   20th  year. 


2  acromial  1 5th  year. 
2  on  vertebral  l)orderl6th 
year. 

Head  1st  year. 
Great  tuberosity  ,3d  year. 
Lesser  tuberosity  5th  year. 
Inner  condyle  5th  year. 
Capitellum  .3d  year. 
Trochlea  10th  year. 
Outer  condyle  14th  year. 
(Olecranon  10th  year. 
Distal  epiphysis  4th  year. 
Proximal    epiphysis    5th 

year. 
Distal  epiphysis  2d  year. 


V  20th  year. 
15th  year. 
I  20th  year. 

>•  17th  year. 

16th  year. 
18th  year. 
17  th  year. 

20th  year.  ' 


.3d  year. 
,3d-5th  years. 


20th  year. 
17th- 18th  year. 


The  dates  in  italics  are  before  birth. 


± 


I'OST-NATAI.    ni-VriOI'MKNT. 


5«5 


LOVVUR  liXTRHMITV. 


Bonk. 


Appkakanck    HI' 

I'RIMAKV    CKN- 
IKK. 


APPKAHANlK  OK  SkiONDAKY 
Cb.N  TKRS. 


Ilium,    3<i  month. 


Isdiiiun, 
l'iil)is,  . 
I'atfUii, 


,W  4//i  month. 
Atii  month. 


Crest  1 3th  year. 
Anterior    inferior    spine 

1 5th  year. 
Tuberosity  15th  year 
Crest  1  «th  year 


KlSloN  OK  Ckn- 
TKKS. 


22(1  year. 


Cart'iia^e"  apiH-arV  at  -ith  month,  ossification  in  .Ul  year. 


I'"eiuur 'th   wrik. 


Tihia, Hthurrl; 


l-il)ula,     ... 

Astragalus, 
Cakaneum, 
Cuboid,    .    . 


lO'ik. 


Scaphoid,    .  . 
Cuneiform,    . 
Metatarsals, 
Phalanges, 


8/// 

7th  month. 
Mh  month. 
A    few    days 
after  birth. 
4th  year. 
1st  year. 
8<fc  u-i-ck. 
Sth-\Oth  urck 


Head  1st  vear 

Creat  trochanter  4th  year 

Lesser    trochanter     1  Mil 

14th  year. 
Condyle  ^)th  month. 

Head'  end  oj  ')th   month. 
Condyle  '>/'(  month. 

Distal  end  2d  vear. 

l^pper  epiphysis  5th  year.     21st  year. 

lower  eniohysis  2d  vear.      20th  year. 


I'Hh  vear. 
l«th  year. 

21st  vear. 
2 1  si   25th  year. 
21  St  vear. 
18th  year. 


Lower  epiphysis 
10th  year. 


.^d  year. 
4th-8th  years. 


16th  year. 


!  20th  vear. 


The  dates  in  italics  are  before  birth 


So  far  as  actual  changes  in  the  form  of  the  appendicular 
bones  are  concerned,  these  are  most  marked  in  the  case 
of  the  lower  Hmb.     The  ossa  inno-ninata  alter  somewhat 
in  their  proportions  after  birth,  a  fact  which  may  con- 
veniently be  demonstrated  by  considering  the  changes 
which  occur  in  the  proportions  of  the  pelvic  diameters, 
although  it  must  be  remembered  that  these  diameters  are 
greatly  influenced  by  the  development  of  the  sacral  curve. 
TaKing  the  conjugate  diameter  of  the  pelvic  brim  as  a 
unit  for  comparison,  the  antero-posterior  (dorso- ventral) 
and  transverse  diameters  of  the  child  and  adult  have  the 
following  proportions  rFehUng) : 


r\'^*tn- 


5i6 


TIIK    lM-:VE[.OI'MKNT    OI"    TIIK    III  MAN    llODV. 


1     I 


DiAMH  tKR. 


Niw  luiRN       Ann  I        Ni-svhdkn       Ann  i 


Kkmai.k.       Kkmai.k. 


Maik. 


Mai  K. 


.  j  Ci)njiiK;>t:i  vtr;i I  00 

£  I  Tran^viTsi' II'' 

■;.,  f  AiiliTd  iMisUTinr,  {\  '>(< 

3  (Ti         .IT^f, Mil 

-  I  AiiHTo  pustiTi'ir,     <•  "M 

5  (.  Tr;ms\  tTsf (I  M  ? 


1    (K) 

)    00 

1  <>o 

1    l'>2 

1   20 

1   2')4 

1  1') 

0   <M 

1    IS 

1  IM 

0   ")'l 

11* 

1    0^ 

0  7,S 

1    (17 

1  m 

0   ,S4 

1   iv; 

It  will  Ik'  seen  from  this  that  the  ut'iu-ral  form  of  thr 
]H-lvis  in  the  now  born  oiiild  is  that  of  a  t-otie,  gradually 
diminishing  in  diamc-tcr  from  the  brim  to  the  outlet,  a 
condition  very  dilTerent  from  what  obtains  in  the  adult. 
Furthermore,  it  is  interestin<;  to  note  that  sexual  differ- 
ences in  the  form  of  the  pelvis  are  clearly  distinguishable 
at  birth;  indeed,  according  to  I'ehling's  observations, 
they  become  noticeable  during  the  frmrtli  month  of  intra- 
uteiine  development. 

The  ui)per  epiphysis  of  the  fenuir  is  entirely  unossitled 
at  birth  and  consists  of  a  cartilagiiums  mass,  much  broader 
than  the  rather  slender  shaft  and  possessing  a  deep  notch 
upon  its  upper  surface  (I'ig.  270).  This  notch  marks  off 
the  great  trochanter  frotn  the  head  of  the  bone,  and  at 
this  stage  of  development  there  is  no  neck,  the  head  being 
])ractically  sessile.  As  development  proceeds  the  inner 
upper  portion  of  the  shaft  grows  more  rapidly  than  the 
outer  portion,  carrying  the  head  away  from  the  great  tro- 
chanter and  forming  the  neck  of  the  bone.  The  acetabu- 
lum is  shallower  at  birth  than  in  the  adult  and  cannot 
contain  more  than  half  the  head  of  the  femur;  conse- 
(juently  the  articular  portion  of  the  head  is  much  less 
extensive  than  in  the  adult. 


I'OS  I-  N  MAI.    I  )IA"  1-I.OlM  KN  r. 


5«7 


It  i^  a  well-known  fact  that  the  new  horn  child  habttu- 
allv  liohls  the  feet  with  the  soles  directed  towanl  one  an- 
other   a  position  o,ilv  reached  in  the  adult  with  sonu- 

aithculty.  and  associate.!  with  '^  ^^f^f;;' ''^  "^'^Z 
there  is  a  pronounced  extension  of  the  foot  (..  ...  tlexum 
upon  the  le«  as  usually  understood;  see  p.  .07).  it  betn,^ 
cliflicult  to  flex  the  child's  foot  beyond  a  line  at  rt.d.t  an.dcs 
with  the  axis  of  the  leK-  These  conditiot.s  are  dtte  appur 
cMitlv  to  the  extensor  and  tibialis  muscles  betn-  relat.vel> 


,p,  Epiphysial  center  for  the  head;  /,.  head  ;  /,  tr..chanter.-(//.«fe^.) 

Shorter  and  the  opposing  muscles  relatively  longer  than  in 
the  adult,  and  uith  the  elongation  or  shortemng  as  the 
case  mav  be,  of  the  nmscles  on  the  assumption  of  the  erec 
position,  the  bones  in  the  neighborhood  ot  the  ankle-jomt 
come  into  new  relations  to  one  another,  the  result  bemg  a 
modification  of  the  form  of  the  articular  surfaces,  espc- 
ciallv  of  the  astragalus.     In  the  child  the  articular  carti- 
lage'of  the  trochlear  surface  of  this  bone  is  continued  on- 
ward to  a  considerable  extent  upon  the  neck  of  the  bone, 
which  comes  into  contact  with  the  tioia  in  the  extreme 


\ 


5i8 


TIIK    I>KV  KI.OI'MKN  r    Oh     TIIF.    IH'MAS    r.<»l>V. 


h 


extension  possible  in  the  ehiUl.  In  the  adult,  however, 
such  <'.\tretne  extension  lieinj;  impossible,  the  eartila)j;e 
upon  tlie  neck  jjradually  disappears.  The  sui)ination  in 
the  child  brink's  the  astrai^alus  in  close  contact  with  the 
inner  suface  of  the  os  calcis  and  with  the  sustentaculum 
tali;  with  the  alteration  of  position  a  jjrowth  of  these  por 
tions  of  the  ealcaneum  occurs,  the  sustentaculum  becom- 
ing; higher  and  broader,  and  so  becoming'  an  obstacle  in 
the  wav  of  supination  in  the  adult.  At  thr  same  time  a 
ijreater  extent  of  the  outer  surface  of  the  astrai;alus  comes 
into  contact  with  the  outer  malleolus,  with  the  result  that 
the  articular  surface  is  considerably  increased  on  that 
l)ortion  of  the  bone.  Marked  chanj^es  in  the  form  of  the 
astrasj;alo-scaphoid  articulation  also  occur,  but  their  con- 
sideration would  lead  somewhat  further  than  seems 
desirable. 

LITERATURE. 

C.   .\EBY:  '  Dif  .Xltersverscliifdcnliciti'n  dcr  imnsclilichen  Wirhelsiiulc," 

Archil:  jur  Anal,  iind  I'h     lol.,  Amil.  Ahtli.,  l.ST'J. 
W.    Camekv:k  ;  "  rrjtcrsucluinni'n    iihtr    Masst-nwaclislliuni    und    I.angen- 

waclistlnuii  (ier  KiiuiiT,"  Juliibiuli  jur  K  iihli  tin  iikumli ,  .XaXVI,  )S'),V 
H.  H.  Donaldson:  "Tlie  Grnwlh  of  tlu-  Hrain,"  Loiuldii,  1«'>3. 
H.  I'EULiNt;:  "Die  Konii  ilcs  Heckcns  ht-im  I'olus  und  .W-ujjehorcnfn  iind 

ilire  He/;ie)mng  zu  uer  l)oiiii   Krwaclistnen,"  Arcliii   jur  (iyuakiil.,  .x, 

1876. 
W    llESKG:    '.KnatoiTiii-  dcs  Kindcrsaltt-rs,"   Iltuulhucit  ih  r  Kituhrkrunk- 
luiUn  (<i,rliardt),  Tiihint,'t.'n,  1881 
C.  Hennk".  ;  "  Das  kindlidio  Hfcken,"  Arcliiv  jiir  Aunt,  inul  I'hysiol.,  Atuit. 

AMh.,  1880. 
C    HiJTEK:  "  Anatuiiiisclie  Stiiditii  an  den   ICxlrciiiitatfnKek'nken  Neugc- 

horencr  und  Hrvvacliscner,"  Arrhiv  jur  /xj/Zm/hi,'.  Aiuit.  uiid  Physiol., 

XXV.  1863. 
W,  Stephe.vso.n :  "On  llic  R-lation  of  Weight  to  Height  and  the  Rate  of 

Growth  in  Man,"  77i<  Laiiai.  ii    1888. 
R.  TiioMA:    "  Untersuchungen  ul)er  die  Grosse  und  dasGewicht  dcr  anato- 

niisciien  Beslandiheilc  dcs  nitiisehlieheit  Kotpi.tr,"  I.eij)zi^,  1882. 
H.   ViEKORUT:  " Analoniische,    Physiologische  und    Pliysikahsciie   Daten 

und  Tabellen,"  Jena,  18(;3. 
n.  WELCKER:  "Untersuchungen  iiber  W'achstlnun   und  Hau  des  nienscli- 

licheT7  Schiidels,"  Leipzig,  1862. 


i     \ 


INDEX 


After -l)irlli,  1  ">'' 
After-bruin,  4(14 
Agger  nasi,  \')'> 
AUantois.  130    IVS 
Alveolo-lingual  Klands,  .UO 

groove,  30<) 
Amitotic  division,  24 
Amnion,  12'' 
Amniotic  cavity,  "I 
Ampliiartliroses,  213 
Amphiaster,  22 
Annulus  of  \icussens,  2S.* 
Anterior  commissure,  426 
Antihelix,  474 
Antitragns,  474 
Antrum  of  Highmore,  199 
Anus,  298 
Aortic  arch,  264 
bulb,  248 
septum,  255 
Apiwndicular  skeleton,  181,  2(Ki 
Archenteron,  65,  296 
Archoplasm,  20 
Arcuate  fibers,  409 
Areas  of  Laiiglians,  332 
Arrectores  pilorum,  168 
Arteries,  261 

allantoidean,  262 
anastomotica  magna.  2  /  6 
anterior  tibial,  2i4,  276 
aorta,  262 
brachioceplialic,  265 
branchial,  262 
carotid,  263 
centralis  retina-,  490 
cceliac  axis,  267 
dorsalis  indicis,  272 
pedis,  274 
pollicis,  272 
epigastric,  271 
cxti'rn:i!  iliac.  275 
facial,  263 
femoral,  276 
hyaloid,  476 


Arteries; 

hyj)ogastric,  268 
iliac,  266 

inferior  mesenteric,  26/ 
innominate,  265 
intercostal,  266 
internal  mammary,  -/ 1 
internal  maxillary,  263 
interosseous,  272 
lingual,  263 
lumbar,  266 
median,  272 
median  sacral,  266 
omphalomesenteric,  242 

peroneal,  276 
j  popliteal,  274 

posterior  tibial,  276 

radial,  272 

saphenous,  275 

sciatic,  274 
1  subclavian,  265,  266 

1  suiTcrior  intercostal,  2<  1 

mesenteric,  262 
I  vesical,  268 

temporal,  263 


ulnar. 


->7? 


umbilical,  262,  267 

vertebral,  271 

vitelline,  242 
Arytenoid  cartilages,  357 
Aster,  21 
Atresia  of  duodenum,  323 

of  pupil,  483 
Auditorv  ganglion,  464 
Auerbach,  i)lcxus  of,  448 
Auricular  septum,  252 
Auriculo-ventricular  valves,  2.->« 
Axial  skeleton,  181 
Axis-cylinder,  396 


Bartholin,  glands  of, 
Belly-stalk,  85,  135 
Bile  capillaries,  327 


3S1 


5>9 


520 


INDFA. 


« 


Bladder,  381 
Blastoderm,  59 
Blastopore,  65 
Blastula,  55 
Blood,  242 
Blood -islands,  241 
Blood-vessels,  240 
Body-cavity,  65 
Bone,  cartilage,  176 

development  of,  1 76 
growth  of,  178 
membrane,  176 
Bone-marrow,  177 
Hones: 

alisplienoid,  197 
atlas,  184 
axis  185 

hasioccjpital,  l'>5 
carpal,  208 
clavicle,  206 
coccyx,  188 
coracoid,  207 
ectethmoid,  197 
ethmoid,  197 
fenmr,  211,517 
fibula,  211 
frontal,  201 
humerus,  208 

hyoid,  204 
ilium,  210 
incus,  203,  470,  472  _ 

innominate,  210,  515 

interparietal,  196 

ischium,  210 

lachrymal,  201 

lingula',  197 

malar,  202 

malleus,  203,  470,  472 

mandible,  204 

maxilla,  203 

mesethmoid,  198 

metacarp.il,  209 

metatarsal,  212 

nasal,  201 

occipital,  195 

orbitosphenoids,  197 

palatine,  203 

parietal,  201 

patella,  211 

Ijhalangcs,  209,  212 

precoracoid,  214 

premaxilla,  203 

presphenoid,  197 

pubis,  210 

radius,  208 


Bones: 

ribs,  183,  185 

sacrum,  188,  509 

scapula,  207 

sphenoid,  196 

st^uamosal,  200 

stapes,  204,  470,  472 

sternum,  188 

supraoccipital,  196 

suprasternal,  189 

tarsal,  211,517 

temporal,  200 

tibia,  211 

turbinated,  199 

tympanic,  200 

ulna,  208 

vertebra-,  181,  510 

vomer,  198 
Bowman,  membrane  of,  497 
Brain,  403 
Branchial  arch  skeleton,  202 

clefts,  91,  101 

epithelial  bodies,  312 

fistula,  94 
Branchiomercs,  123 
Burdach,  colunm  of,  403 

i 

c. 

Ca-cum,  323 

Calcar,  423 

Calcarine  fissure,  423 

Callosal  fissure,  427 

Calloso-marginal  fissure,  424 

Canalis  reuniens,  462 

Carotid  gland,  448 

Cartilage  bone,  177 

Caruncula  lachrynialis,  495 

Cauda  equina,  401 

Cavernous  sinus,  277 

Cell,  17 

Cell-theory,  17 

Cenlrosome,  20 

Cerebellum,  410 

Cerebral  convolutions,  422 
cortex,  428 
hemispheres,  408,  418 

Chin  ridge,  105 

Chondrocranium,  192,  195 

Chorda  dorsalis,  1 1 5 
endodemi,  115 
;  Chorda'  tcndinea?,  258 
I  Chorioid  coat  of  eye,  478,  495 
!  plexus,  407,  416,  421 

Churioidal  fissure,  421,  476 


INDEX. 


521 


Chorion,  84,  142 

frondosum,  145 

lave,  145 
Chromafime  cells,  392 
Chromatin,  20 
Chromosomes,  22 

reduction  of,  32 
Ciliary  body,  485 

gangUon,  444,  448 
Circumvallate  papilla',  458 
Cleft  palate,  203 

sternum,  190 
Clitoris,  385 
Cloaca,  296 

Cloaca!  membrane,  297 
Cloquet,  canal  of,  494 
Coccygeal  ganglion,  452 
Cochlea,  462,  467 
Ccelom,  65 
Collateral  eminence,  425 

fissure,  425 
Coloboma,  483 
Colon,  321 

Columnae  corneae,  258 
Concrescence,  75 
Conjunctiva,  497 
Connective  tissue,  174 
Cornea,  478,  495 
Corniculae  laryngis,  357 
Corona  radiata,  36,  376 
Coronary  sinus,  252 
Corpora  albicantia,  4J7 

quadrigemina,  414 
Corpus  albicans,  40 
callosum,  426 
luteum,  40 
striatum,  420 
Corti,  organ  of,  464 
Cotyledons,  145 
Cowper,  glands  of,  384 
Cricoid  cartilage,  357 
Crista;  acustica?,  464 
Crura  cerebri,  413 
Cuneiform  cartilages,  357 
Cutis  plate,  122 
Cytoplasm,  20 


D. 

Darwin's  tubercle,  475 
Decidua  reflexa,  152 

serotina,  153 

vera,  150 
Deciduae,  128,  147,  159 

44 


Dendrites,  396 
Dental  groove,  300 

papilliE,  300 

shelf,  300 
Dentate  gyrus,  423,  427 
Dentine,  304 
Dermatome,  122,  164 
Diaphragm,  338 
Diaphysis,  178 
Diarthrosis,  213 
Diencephalon,  404,  415 
Discus  proligerus,  35,  376 
Dorsal  flexure,  91 

zone,  400 
Duct  of  Santorini,  332 

j)f  Stenson,  309 

of  Wharton,  309 

of  Wirsung,  332 
Ductus  arteriosus,  264 

Botalli,  264 

communis  choledochus,  325 

Cuvieri,  277 

ejaculatorius,  377 

venosus,  281 
Duodenum,  320 


B. 

Ear,  459 

Ebner,  glands  of,  459 
Ectoderm,  64 
Embryonic  disc,  71 
PJnamel,  301 
Endocardium,  248 
Endoderm,  65 
Endolymphatic  duct,  460 
Enveloping  layer,  59 
Ependymal  cells,  395 
Epiblast,  64 

Epibranchial  ganglia,  440 
Epidermis,  161 
Epididymis,  377 
Epiglottis,  356 
Epiphyses,  178 
Epiphysis  cerebri,  415 
Episternal  cartilages,  189 
Epitrichium,  161 
Eponychium,  166 
Epoiiphoron,  379 
Erythroblasts,  245 
Erythrocytes,  241 
Erythroplastids,  245 
Eustachian  tube,  472 
valve,  253 


I 


if 


522 

Extrauterine  pregnancy,  38 
Kye,  476 
Eyelids,  497 

F. 

illopian  tubes,  .U9 
Fasciculus  communis,  43() 

solitarius,  426 
Fenestra  ovalis,  469 

rotunda,  4?.  - 
Fertilization  .i  ovum,  4/ 
Fetal  circulation,  2ii8 
Fifth  ventricle,  427 
Filum  terminale,  401 
Fimbria  ovarica,  379 
Flocculus,  4 1 1 
IMoor-platc,  400 
Foliate  papillse,  459 
Fontana,  spaces  of,  496 
Foramen  ca-cum,  306 
incisivum,  300 
of  Winslow,  343 
ovale,  253 
Fore-brain,  404 
Formatio  reticularis,  409 
Fornix,  426  . 

Fossa  supratonsillans,  M^ 
Fnmtal  sinuses,  199 
Furcula,  31 1 


Gall-bladder,  325 
GanKli<mited  cord,  445 
Oiirtner,  canals  of ,  379 
Gastral  mesoderm,  67 
Gastrula,  64 
Geniculate  bodies,  417 
Genital  folds,  384 
ridKC,  360,  371 
swellings,  385 
tubercle,  384 
Germ  cells,  24 
plasma,  25 
Germinal  layers,  64, 
Giant  cells,  247 
Giraldes,  organ  of,  377 
Goll,  column  of,  403 
Graafian  follicle,  34,  375 
Gray  rami,  444 
Growth  of  body,  502 
Gubernaculum  testis,  371 
Gvnaecomastia,  173 


78 


1NM>EX. 


Gyrus  fornicatus,  424 
niarginalis,  424 


H. 

H;cmatopoictic  organs,  244 
Hair,  167 
Harelip,  105 
Haversian  canals,  180 
Head  l>end,  ')4 

cavities,  120,_440 
j)rocess,  74,  76 
Heart,  248 
Helix,  474 
Hensen's  node,  74 
Hermaphroditism,  387 
I  Highmore,  antrum  of,  199 
j  Hind-brain,  404 
I  Hippocam  pal  fissure,  423 
I  Hippocampus,  423 
I  minor,  423  ^ 

:   Holoblastic  segmentation,  . 

Hyali>i<l  canal,  494 
'   Hydatid  of  Morgagni,  378 
i  stalked,  381 

'  Hymen,  380 

Hvpcrthelia,  173 
i  Hypertrichosis,  169 
Hypoblast,  65 
Hypochordal  bar,  183 
Hypophysis  cerebri,  418 
Hypospadias,  387 


I. 

Infundibulum,  420 
Inguinal  canal,  390 
Insula,  424 

Interarticular  cartilages,  214 
Intercarotid  ganglia,  448 
Intermediate  cell  mass,  119 
Intermuscular  septa,  182 
Intervertebral  discs,  184 
Intestine,  319 
Intraparielal  fissure,  424 

Iris,  485 
Isthmus,  404,  413 

Iter,  405 


Jacobsim,  organ  of,  45/ 
Joints,  212 


INDEX. 


523 


Karyokinesis,  24 

Karyoplasni,  20 

Kidney  (see  Mitancphros),  366 


Labia  majora,  385 

minora,  385 
Lachrymal  duct,  499 

gland,  498 
Lamina  spiralis,  468 
terminalis,  418 
Lancisi,  stria;  of,  427 
Langlians,  areas  of,  3-' , 

cells  of,  145 
Lanugo,  168 
Larynx,  355 
Lateral  sinus,  277 

thyreoids,  314 
Lens,  476,  479 
Lenticular  ganglion.  444 
Leukocytes,  243 
Ligaments: 
broad,  371 
capsular,  213 
coronary,  340 

external  lateral,  of  knee,  221 
great  sacro-sciatic,  221 
infraspinous,  184 
inguinal,  388 
intervertebral,  184 
ovarian,  371 
pectinatum  iridis,  495 
round,  of  liver,  290 
spheno-mandibular,  204 
subtlavan,  184 
supraspinous,  184 
suspensory,  of  lens,  493 
suspensory,  of  liver,  340 
teres,  of  ovary,  371 
Limbs,   105 
Lip  ridge,  105 
Lips,  299 
Lifiuor  amnii,  133 
Liver,  325 
Lungs,  352 
Lunula,  166 
Luschka's  ganglion,  452 
Lymph  hearts,  291 

nodes,  294 
Lymphatics,  291 
Lymphocytes,  246,  291 


Maculae  acusticoe,  464 

Mammary  glands,  170 

Mandibular  process,  97 

Mantle  layer,  395 

Marchand,  accessory  suprarenals  of, 

391 
Mastoid  cells,  472 
process,  200 
Maturation  of  ovum,  43 
Maxillary  process,  97 
Meatus  auditorius  externus,  473 
Meckel's  cartilage,  194 

diverticulum,  135   323 
Mediastina,  341 
Medulla  oblongata,  404 
Medullary  canal,  1 14 
fold^^    36,  112 
groove,  1 12 
sheath, 399 
Megacaryocytes,  247 
Meibomian  glands,  497 
Meissner,  plexus  of,  448 
Membrana  pupillaris,  483 
reuniens,  123 
tectoria,  465 
Membrane  bone,  1 76 

Menstruation,  38 

Meroblastic  segmentation,  57 

Mesencephalon,  404,  414 

Mesenchyme,  80 

Mesenteriole,  347 

Mesentery,  342 

Mesocardium,  334 

Mesocolon,  345 

Mesoderm,  65 

Mesoderniic  somites,  89,  118 
I  Mesogastrium,  342 
I  Mesonephros,  363 
I  Mesorchium,  371,  389 

Mesothelium,  80 

Mesovarium,  371 

Metamere,  124 

Metanephros,  366 

Metencephalon,  404,  410 

Metoi)ic  suture,  201 

Midbrain,  404 

Middle  commissure,  416 

Milk  ridge,  1 70 

Mitosis,  24 

Moderator  bands,  258 

M(j11,  glands  of,  497 

Monro,  foramen  of,  4::0 
sulcus  of,  415 


l. } 


i   1    t 


524 

Mt'nstrosilics,  63 

Morgagni.  hydatid  of,  378 
Morula,  59 
Mouth  cavity,  299 
Miillerian  duct,  369 
Muscle  plate,  122 
Muscles : 

biceps  feinoris,  221 

branchioineric,  225 

chondroglossHS,  231 

ciliary,  496 

coccygeus,  225 

constrictorcs    pharyngis,    229, 

231 
cranial,  227 
curvator  coccygis,  225 
digastric,  229 
dilatator  iridis,  486 
dorsal,  223 
erector  spina?,  220 
external  rectus,  228 
gastrocnei'iius,  237 
gcniohyog  ossus,  224 
geniohyoic ,  224 
hyoglossus,  224 
hyposkeletal,  224 
intercostal,  220,  224 
laryngeal,  229 
latissinius  dorsi,  2 1 9 
levator  ani,  225 
limb,  231 
longus  colli,  224 
masseter,  229 
mylohyoid,  229 
obliqui  abdominis,  220,  224 
occipito-frontaHs,  221,  229 
omohyoid,  220 
palatoglossus,  231 
perineal,  227 
peroneus  longus,  221 
platysma,  229 
psoas,  224 
])terygoid,  229 
pyramidalis,  224 
rectus  abdominis,  220,  224 
scaleni,  224 
serrati  postici,  221 
serratus  magnus,  220 
skeletal,  218 
soleus,  237 
sphincter  ani,  225 
s])hincter  cloacx,  225 
sphincter  iridis,  486 
stapedius,  229,  470 


INI>KX. 


Muscles; 

sterno-niastoid.  220,  224,  231 
styloglossus,  224 
stylohyoid,  229 
stylopharyngeus,  229 
sui)erior  oblique,  227 
temporal,  229 
tensor  palati,  229 
tensor  tympani,  229,  470 
transvcrsus  abdominis,  220,  224 
trapezius,  220,  224,  231 
triangularis  sterni,  224 

Muscular  tissue,  216 

Musculi  papillares,  258 

Myelence])halon,  404,  407 

Myelin,  399 

Myocardium,  248 

Myotome,  122 

N. 

i  Nail  fold,  165 
Nails,  164 
Nasal  duct,  499 
fossa*,  97 
process,  104 
Neck  bend,  94 

depression,  98 
Nephrostoine,  362 
Ncphrotome,  120 
Nerve  roots,  397 
Nerves : 

cranial,  430 

hypoglossal,  434 

olfactory,  455 

()l)tic,  489 

recurrent  laryngeal,  358 

spinal,  429 

accessory,  429,  438 
spino-occipital,  439 
s])lanchnic,  446 
superior  laryngeal,  358 
Nervous  system,  394 
Neural  aich,  183 

ridge,  397 
Neurenteric  canal,  86,  1 12 
Neuroblasts,  395 
Neuroglia,  395 
Neuronieres,  440 
Neurone  theory,  399 
Non-sexual  reproduction,  25 
Notochord,  115 
Nuck,  canal  of,  388 
Nucleoli,  20 
Nucleus,  19 


INDEX. 


525 


Occipital  depression,  98 
Odontoblasts,  .504 
(Hsophagus,  317 
Olfactory  lobes,  427 
Olivary  body,  409 
Omentum,  greater,  343,  347 

lesser,  343 
Oiicyte,  43 
Optic  cup,  476,  483 

thahmii,  416 
Ova  serrata,  484 
Oval  fossa,  90,  103 
Osteoblasts,  176 
Osteoclasts,  180,  247 
(Jtic  ganglion,  444,  448 
Otocyst,  460,  476 
Ovaries,  descent  of,  387 
Ovary,  374 
Ovulation,  37 
Ovum,  33,  376 

fertilization  of,  47 
maturation  of,  43 
segmentation  of,  53 


P. 

Palate,  299 
Pancreas,  331 
Paradidymis,  377 
Paraphysis,  416 
Parathyreoids,  3  4 
Parietal  cavity,  335 
Parieto-occipital  fissure,  423 
Paro(")plioron,  379 
Parovarium,  379 
Parthenogenesis,  25 
Penis,  385 

Pericardial  cavity,  338 
Perilymph,  467 
Perineal  body,  384 
Perionyx,  166 
Periosteum,  176 
Periotic  capsule,  192,  20y> 
Peritoneum,  342 
Petit,  canal  of,  494 
Petrosal  sinus,  277 
Pfluger's  cords,  375 
Pharyngeal  bursa,  312 

membrane,  297 

tonsil,  311 
Pharnyx, 310 
Pineal  body,  415 
Pinna,  474 


Pituitary  body,  419 
Placenta,  155 

foetalis,  155 

praevia,  155 

uterina,  155 
Pleural  cavities,  341 
Pleuro-peritoneal  cavity,  1 20,  338 
Plica  semilunaris,  498 
Polar  globules,  43 
Polycaryocytes,  247 
Polymastia,  173 
Polyspermy,  49 
Post-anal  gut,  297 
Post-branchial  bodies,  316 
Post-central  fissure,  424 
Posterior  root  ganglia,  347 
Precentral  fissure,  424 
Prepuce,  386 

Primitive  streak,  67,  68,  74 
Processus  globularis,  104 
Pronephric  duct,  361 
Pronephros,  361 
Pronuclei,  49 
Prostate  gland,  384 
Prostomial  mesoderm,  67 
Protoplasm,  18 
Protovertebrx,  118 
Pulvinar,  417 


R. 

Rathke's  pouch,  300 
Rauber's  covering  layer,  62 
Receptaculum  cliyli,  2'M 
Recessus  parietales,  335 
Rectum,  297 
Red  nucleus,  414 
Reil,  island  of,  424 
Restiform  body,  410 
Rete  ovarii,  376 

testis,  374 
Retina,  486 
Rhinencephalon,  428 
Rolando,  fissure  of,  424 
Roof-plate,  400 
Rosenniiiller,  groove  of,  312 

organ  of,  379 


S. 

Sacral  bend,  94 
Salivary  glands,  309 
Santorini,  cartilages  of,  357 
duct  of,  332 


526 

Sarcode,  18 
Sclerotic  coat,  478,  495 
Sclerotome,  122 
Scrotum,  386 
Sebaceous  glands,  168 
Segmentation  nucleus,  49 

of  ovum,  54 
Semicircular  canals,  461 
Semilunar  valves,  259 
Seminiferous  tubules,  374 
Septum  luciduni,  426 
transversum,  336 
Sertoli  cells,  30 
Sexual  reproduction,  25 
Sinus  pocularis,  379 
praccervicalis,  101 
terminalis,  241 
venosus,  248 
Situs  inversus  viscerum,  63 
Skelet<m,  181 
Skin,  101 
Skull,  191,  511 
Socia  j)arotidis,  309 

Soft  commissure,  416 
Solar  plexus,  446 

Sole  plate,  165 

Solitary  fasciculus,  408 

Somatic  cells,  24 
mesoderm,  120 

Spermatic  cord,  390 

Spermatid,  30 

Spermatocyte,  30 

Spermatogenesis,  29 

Spermatogcmia,  30 

Spermatozoa,  27 

Sphenoidal  cells,  219 

Sphenojjalatine  gangli(»n,  444,  448 

Sjjinal  cord,  400 

Splanchnic  mesoderm,  120 

Spleen,  349 

Stenscm'sduct,  309 

Sternum,  cleft,  190 

Stomach,  318 

Stratum  granulosum,  35,  376 

Sublingual  ganglion,  448 
gland,  310 

Submaxillary  ganglion,  444,  448 
gland,  309 

Substance  islands,  241 

Subthalamic  region,  417 

Sudoriparous  glands,  169 

Sulcus  of  Monro,  415 

Superfetation,  52 

Superior  longitudinal  sinus,  277 

Suprabranchial  ganglia,  440 


INDEX, 


Suprarenal  bodies,  390 
Suprarenals,  accssory,  391 
Suture,  212 
Sylvian  fissure,  424 

fossa,  423 
Symi)athetic  system,  441 
vSynchondrosis,  212 


Tail  fdament,  98 
Taste,  organs  of,  458 
Teeth,  300 
Tegmentum,  413 
Telencephalon,  404,  418 
Temporal  fissures,  424 

lobe,  421 
Testes,  descent  of,  388 
Testis,  372 

Thalamencephalon,  404 
Thebesian  valve,  253 
Thoracic  duct,  291 
Thymus  gland,  315 
Thyreo-gli)SS;il  duct,  314 
Thyreoid  body,  313 

cartilage,  356 
T:>nguc,  305 
Tonsils,  312 
Touch,  organs  of,  458 
Trachea,  355 
Tragus,  474 
Trophoblast,  72 
Tuber  cinereum,  417 
Tuberculum  impar,  305 
Tubuli  recti  testis,  374 
Tunica  albuginea,  372 

vaginalis,  389 

vasculosa  lentis,  482 
'Tween-brain,  404 
Twins,  63 
Tympanic  cavity,  469 

membrane,  474 


U. 

Umbilical  cord,  97,  139 
Umbilicus,  87 
Urachus,  138,382 
Ureter,  366 
Urethra,  383 
I'rinogcnital  system,  360 
Urogenital  sinus,  382 
Uterus,  379 

masculinus,  379 


INDEX. 


527 


Utriculus,  462 
Uveti,  484 


Vagina,  379 
Vaginal  process,  388 
Vas  a'iH.Trans,  377 
deferens,  377 
Veins : 

anterior  tilnal,  288 
ascending  luinliar,  286 
a/}  4  'S,  286 
basiiic,  287 
cardinal,  280 
cephalic,  287 
emissary,  280 
external  jugtilar,  280 
facial,  280 
hemiazygos,  286 
hepatic,  283 
inferior  cava,  284 
innominate,  279 
internal  jugular,  276 
jugulo-cephalic,  288 

omi)luilo-mesenteric,  242,  281 

ovarian,  28'> 

portal,  282 

renal,  285 

sciatic,  288 

spermatic,  285 

suhcardinal,  28  t 

su|)erior  cava,  279 

suprarenal,  284 

umbilical,  281 

vitelline,  242 
Velum,  anterior,  413 

interpositum,  416 

marginal,  395 

posterior,  407 
Ventral  zone,  400 


Ventricle,  fourth,  405 
lateral,  405 
third,  405 
Ventricular  septum,  254 
Vermiform  appendix,  324 
Vermis,  411 

Vernix  caseosii,  134,  168 
Veru  montanum,  380  ^ 
Vesicula;  seminales,  377 
Vieussens,  annulus  of,  253 

valve  of,  413 
VilH,  chorionic,  143 

intestinal,  3J4 
\itrcous  humor,  492 
Vocal  cords,  356 
Vidva, 385 

W. 

Wharton's  duct,  309 

jelly,  141 
White  rami,  443 
VVirsung's  duct,  332 
Witch  milk,  173 
Wolffian  body,  361 

duct,  361 

ridge,  360 
Wrisberg,  cartilages  of,  357 

Y. 

Volk-Siic,  83,  130,  134 
Yolk-stalk,  87,  130 
Yolk-vesicle,  87 


Zima  pellucida,  36 
Zonula  Zinnii,  493 
Zuckerkandl,  organs  of,  450 


